Ceremonies have been held in South Korea to mark the 60th anniversary of the outbreak of the Korean War.
In Seoul, President Lee Myung-bak paid tribute to veterans and called for peace on the peninsula.
But he also urged North Korea to stop its "reckless" behaviour, amid high tension over a sunken warship.
North Korean tanks and troops swept south on 25 June 1950, starting a three-year conflict that claimed as many as four million lives.
The US led the United Nations support for the South while China and Moscow backed the North.
The war ended with a ceasefire rather than a peace treaty, leading to almost constant political tension ever since.
North Korea still disputes the widely-held view that it attacked first.
The BBC's John Sudworth in Seoul says a number of commemorations are being held, many attended by veterans from overseas.
But with tensions on the peninsula still high there is little to celebrate, our correspondent adds.
Seoul and Pyongyang are currently locked in a dispute over the sinking of a South Korean warship near their disputed maritime border.
Forty-six South Korean sailors died when the Cheonan went down on 26 March.
An international inquiry found a North Korean torpedo sank the ship, but North Korea strongly denies this.
In Friday's ceremony, Mr Lee urged North Korea to stop "reckless military provocation" and "clearly and honestly admit its wrongdoings" in the sinking of the warship.
South Korea's goal was "not a military confrontation but peaceful reunification", he said, calling on Pyongyang to take a "responsible attitude".
Foreign service personnel and diplomats joined thousands of South Korean veterans at the main ceremony in Seoul.
Flags from the 20 countries that sent troops or medical staff to back South Korea under the UN flag were also flying.
North Korea has also marked the anniversary in state media.
On Tuesday it reaffirmed its position that the US provoked the war. On Thursday Pyongyang demanded $65 trillion (£43.5 trillion) in reparations from Washington.
It has also announced a temporary ban on shipping off a section of its west coast - something which has in the past preceded a short-range missile test.
"This appears to be part of training exercises and we have no indications of unusual activities by the North Korean military," a statement from the South Korean Defence Ministry said.
South Korea is pushing for a UN resolution condemning Pyongyang over the Cheonan incident - something that North Korea has warned would be unacceptable.