Seizing on the momentum built up by his historic summit with the North's leader Kim Jong-Il, the president said in a speech to thousands of war veterans that any new conflict would decimate the whole Korean peninsula.

He said the 85,000 US troops in South Korea and Japan must remain and that North Korea now understands this.

The 10,000 strong crowd at the Seoul War Memorial, including 4,500 Korean and foreign veterans, applauded the president's call for strict vigilance against the North.

"Peace can only be guaranteed through a tight defence posture. I emphasize that we should not allow the slightest gap in our defence posture," he said.

The president said he had stressed to Kim Jong-Il that another war would be radically more destructive than the 1950-53 conflict that has never been formally ended with a peace treaty.

The Korean War started on June 25, 1950 when North Korean troops poured across the border and soon took Seoul.

More than three million soldiers and civilians died in the conflict which ended in a deadlock with the two sides grouped on either side of the De-Militarized Zone that divides the peninsula.

There are now widespread fears over North Korea's missile and nuclear programmes.

"We should never dream of the unification by force or absorbing the other side," the president said. "We have to let the 70 million Koreans to live free from the fear of war."

The June 13-15 summit in Pyongyang between the Kims produced a landmark accord to take steps toward reconcilation. That in turn has completely changed attitudes in the South about its isolated Stalinist neighbour.

Because of the breakthrough, South Korea cancelled many war commemoration events, including the main veterans' parade. North Korea also agreed to tone down its ceremonies and diplomats contacted in Pyongyang said no official events were noticed.

North Korea's strictly controlled state media remained silent on the anniversary. Seoul monitors said there was no mention of the war in the Rodong Shinmun, the ruling party's paper, unlike preceding years.

Ceremonies were held and prayers said across South Korea. But in Seoul, about 1,000 dissidents, radical students and union members scuffled with police as they tried to get near the US embassy for a rally to demand the withdrawal of US troops.

Kim Dae-Jung said: "The only road to national survival is through peaceful co-existence, peaceful exchanges, and peaceful unification. The North expressed their shared feelings on this matter."

The president said the Korean War had been part of a communist plan to take over the whole Asia-Pacific region and gave strong backing to the 37,000 US troops in South Korea and 48,000 based in Japan.

"The motive of the Korean War was not merely to communize South Korea. It was a part of the communist plan to control the whole Asia-Pacific, including Japan which had been weakened substantially at that time."

Kim Dae-Jung said he had emphasised South Korea's support for US troops during his talks with Kim Jong-Il.

"I explained to the North that the US armed forces will stay until a complete peace system is put in place on the Korean peninsula," said Kim Dae-Jung, who added: "The North showed substantial understanding on my explanation on the need for the US troops."

The president said that if US troops left South Korea and Japan, "the security and balance of power on the Korean peninsula and in East Asia and the Pacific will be seriously affected."

The United States hopes to resume talks this week with Pyongyang officials on North Korea's missile programme, according US officials. The programme is considered a major threat to Japan and eventually the United States.

The president said: "We will continue to maintain firm coordination with the United States and Japan. At the same time, we will keep close partnerships with China and Russia.