Thailand's military junta said Saturday it had disbanded the Senate and placed all law-making authority in the hands of the army chief, dramatically tightening its grip after a coup that has sparked Bangkok protests and drawn international condemnation.

The regime also confirmed it had detained former premier Yingluck Shinawatra and scores of other ousted government leaders and would hold them for up to a week as it corralled potential opponents to its takeover.

"The Senate is dismissed. Responsibility for any laws needing the approval of the parliament or Senate will instead be assumed by the leader of the (junta)," said an army bulletin on national television.

Late Saturday, the Pentagon cancelled an ongoing military exercise as well as planned visits by officials to the country.

"While we have enjoyed a long and productive military-to-military relationship with Thailand, our own democratic principles and US law require us to reconsider US military assistance and engagements," spokesman Rear Admiral John Kirby said in a statement.

The cancellations came after Washington, which has led criticism of the coup, suspended $3.5 million (2.6 million euro) in military assistance to its ally -- about one-third of its Thai aid.

Thailand's fragile democracy has been repeatedly stunted by 19 actual or attempted coups since 1932.

Analysts called Saturday's developments an ominous sign that the junta led by the army chief, General Prayut Chan-O-Cha, could be digging in for a long-term, pervasive takeover, as it had earlier said the Senate would be retained.

Sporadic protests flared in Bangkok for a second day, with hundreds of demonstrators defying a ban on political gatherings to denounce the coup, echoing international calls for the restoration of civilian rule.

Prayut seized control Thursday after anti-government forces had waged a several-month campaign to oust Thailand's civilian leaders.

Civil liberties have been curbed, media restrictions imposed, most of the constitution abrogated, and rival protesters from both sides of the political divide cleared from the capital.

- King 'acknowledges' coup -

Political analysts view the coup as part of a long-running effort by a Bangkok-based power elite -- aligned with the monarchy and military -- to eliminate the political dominance of Yingluck's elder brother Thaksin Shinawatra.

Thaksin, a billionaire telecoms tycoon, shook up Thai politics by winning devotion among millions of rural poor with populist measures, catapulting him to prime minister in 2001 polls.

He was deposed in 2006 in a military coup, fleeing abroad two years later to avoid a corruption conviction, but his family and allies have continued his success at the ballot box.

The military said Saturday that Prayut had sent a letter regarding his takeover to the country's revered king, Bhumibol Adulyadej.

The monarch, 86, commands great respect among his subjects, and his blessing is traditionally sought to legitimise Thailand's recurring military takeovers.

The army said the king had "acknowledged" Prayut's letter, but stopped short of describing the response as an endorsement.

The palace has yet to issue a statement on the crisis.

Pavin Chachavalpongpun, a Thai politics researcher at Japan's Kyoto University, said the general's assumption of law-making authority indicates a looming "full-scale authoritarianism or military despotism."

"By taking full control of the executive, legislative and judicial branches, and now the Senate, this shows the military is setting itself up as the sovereign," he said.

Briefing the media for the first time since the coup, the military said Yingluck and other prominent figures had been taken in under a martial law provision allowing detentions of up to seven days without charge.

- Yingluck detained -

It was not known whether any charges were being prepared, but military officials said the detainees were in no danger.

"(Yingluck) is under detention, and she is fine," Lieutenant General Thirachai Nakwanich, head of the military command for central Thailand including Bangkok, told AFP.

A military official told AFP she was being held at an unspecified army barracks in Bangkok.

Anti-Thaksin forces -- alleging corruption in Yingluck's administration -- have staged months of deadly Bangkok protests that led to at least 28 deaths and triggered counter-demonstrations by the pro-Thaksin "Red Shirt" movement.

Tensions spiralled in early May when a court ruling controversially stripped Yingluck of office, setting the stage for the coup.

The "Red Shirts" had earlier warned a coup could trigger civil war but are yet to mount a clear response.

However, for the second straight day, demonstrators gathered in at least two locations in central Bangkok to rail against the junta.

"I have the right to vote and I want to make it clear that they cannot just steal it from me with a coup," said a tour guide who identified himself only as Piti. The protesters in Bangkok said they were not Red Shirts.

A night-time curfew imposed by the junta has tamed the capital's tourist districts, further chilling a vital visitor industry already cooled by the long-term strife.

No serious confrontations have yet been reported, but AFP reporters witnessed several people being taken into custody in various locations.

International alarm mounts over Thai coup
Hong Kong (AFP) May 23, 2014 - Allies and key trading partners expressed disquiet over Thailand's coup Friday, demanding a quick return to civilian rule as some warned against travel to the prime tourist destination.

Japanese auto giants that have invested heavily in Thailand were forced to stop night operations at their factories to comply with a curfew imposed by the new junta, which seized power on Thursday in a move the United States said had "no justification".

Japan, by far Thailand's biggest foreign investor overall, described the coup as "regrettable".

"Our country wants to call strongly for a prompt restoration of a democratic political system," added Chief Cabinet Secretary Yoshihide Suga.

Toyota, the world's number one carmaker which produces 670,000 cars a year in Thailand, said the curfew had halted production at all three of its local assembly plants late Thursday.

Honda Motor also curtailed operations at its plant on Thursday, stopping four hours ahead of its original planned shutdown at midnight, a spokeswoman said.

But the military regime had granted permission to Toyota for Friday's night shift to go ahead, the Japanese firm said.

The Pentagon said it was reviewing military cooperation with America's oldest Asian ally, while Secretary of State John Kerry warned of potential fallout.

"While we value our long friendship with the Thai people, this act will have negative implications for the US-Thai relationship, especially for our relationship with the Thai military," he said.

India's foreign ministry spokesman Syed Akbaruddin also voiced concerns about the situation.

"Some Indian troops who had gone there for joint exercises have been advised to return back to India," he said.

"We hope for an urgent political resolution and restoration of normalcy based on principles of democracy, will of people and rule of law in Thailand."

In Beijing, foreign ministry spokesman Hong Lei told reporters: "China and Thailand are friendly neighbours. We hope to see normal social order to be restored as soon as possible in Thailand."

The European Union, another of Thailand's top trading partners, expressed "extreme concern" and demanded a rapid return to democracy, while Australia said it was "gravely concerned" at the army's seizure of power.

"It is a volatile situation. We are monitoring it closely but people need to pay close attention to their personal security and their travel plans," Foreign Minister Julie Bishop told ABC radio.

- 'Unpredictable and volatile' -

With Thailand drawing just over 26.7 million visitors last year, according to its tourism authority, foreign governments have largely cautioned nationals to keep their heads down without explicitly advising against travel.

However, key trade partners Malaysia and Hong Kong warned against non-essential trips after months of political turmoil escalated into the military crackdown.

Neighbouring Malaysia -- Thailand's second-highest source of visitors last year with 2.99 million tourists, behind China's 4.7 million -- advised its nationals to postpone any non-essential visits and those there to abide by the curfew.

Hong Kong raised its travel alert for Thailand to its second-highest level. The city's Travel Industry Council said all group tours would be cancelled from May 24 to May 30, affecting 1,300 people.

Singapore also weighed in, warning that the situation was "unpredictable and volatile, and may evolve quite rapidly".

"Singaporeans should seriously reconsider visiting Thailand at the moment," the foreign ministry said.

Medical and travel security risk services provider SOS International also urged business travellers to defer non-essential trips, warning that all gatherings should be avoided due to a "credible risk of violence".

Indonesia, Southeast Asia's largest economy, said it was following developments "with deep and profound concern" and that it would seek the intervention of the Association of Southeast Asian Nations, of which Thailand is a leading member.

ASEAN is traditionally loath to get involved in its members' internal affairs. But Indonesia stressed that the bloc's charter "emphasizes adherence to democratic principles and constitutional government", and so the coup merited engagement by the region.