South Korean police on Monday blocked activists from sending anti-Pyongyang leaflets across the border with North Korea, which had threatened to respond with a "merciless" military strike.

The decision to shut down the propaganda exercise was an unusual one and reflected, analysts said, Seoul's desire to avoid any destabilising clash ahead of South Korea's presidential election in December.

North Korean defectors who had planned to launch balloons carrying 200,000 propaganda leaflets across the heavily-militarised border were infuriated by the move, accusing President Lee Myung-Bak of capitulating to the North's threats.

There were some minor scuffles as around 80 activists sought to push their way through a large roadblock of police vehicles and security personnel set up about four kilometres (2.5 miles) south of the launch site north of Seoul.

"This event has been authorised by the government. This is ridiculous," said Park Sang-Hak, one of the organisers.

"We are not here to provoke a conflict but to convey the truth to North Koreans. President Lee will be remembered as a cowardly leader who succumbed to North Korean threats," Park said.

A small group of about a dozen defectors later claimed to have launched some balloons with leaflets from another site on the western island of Gangwha, near the border with North Korea, Yonhap news agency reported.

It was not possible to independently verify the claim.

Local police officials said the decision to block the activists had been dictated by "security concerns" after the North Korean army threatened a "merciless military strike" if the event went ahead.

"The surrounding area will become targets of direct firing," the Korean People's Army said in a statement.

China and the United States both stressed the need for further restraint, with Washington denouncing Pyongyang's threat to shell a civilian area.

"It is grossly disproportionate to have threatened to respond to balloons with bombs," US Special Envoy for North Korea Policy Glyn Davies told reporters in Beijing.

Chinese foreign ministry spokesman Hong Lei urged the two Koreas to stay calm and avoid both "provocative" acts and "over-reaction".

North Korea has threatened strikes in the past, but Friday's statement was unusually strong with its specific naming of the time and location, coupled with a warning to local residents to evacuate the area.

It was also the first time such a precise threat had been made under North Korean leader Kim Jong-Un, who took over the reins of the isolated state after the death last December of his father, Kim Jong-Il.

Troops in the South had been placed on high alert and Yonhap news agency reported the deployment of additional artillery and tank units to forward border positions.

President Lee has taken a hard line with North Korea during his five years in office, and the decision to ban Monday's event took some by surprise.

"Stability is the number one priority for Seoul right now," said Yang Moo-Jin, a professor at the University of North Korean Studies who cited the proximity of December's presidential poll as a likely factor.

"I think the president felt that if he let tensions further escalate, he would see his political legacy tarnished at the end of his term and be blamed for leaving a diplomatic burden on the incoming administration," Yang told AFP.

Cross-border tensions have heightened in the run up to December's election, amid widespread concern in the South that North Korea would seek to provoke a clash ahead of the ballot.

Pyongyang had reacted angrily to the announcement two weeks ago of a new US-South Korean agreement to nearly triple the range of the South's missile systems, bringing the whole of North Korea within range.

Seoul's Unification Ministry, which handles cross-border affairs, called Monday for restraint from both Pyongyang and the activists in the South.

"We urge the North to stop the threats... and have constantly asked the civic groups to refrain from such (propaganda) acts, in consideration of inter-Korea relations," a ministry spokesman told reporters.

Civic groups in the South regularly drop leaflets over the border with messages criticising the Kim dynasty and urging the North Korean people to rise up against repression.

The leaflets also carry news about rebellions in other parts of the world, including events of the "Arab Spring".