North Koreans are in mourning after the death of their leader, Kim Jong-il.

People wept openly on the streets of the capital, Pyongyang. State media said he had suffered a heart attack on Saturday, aged 69. He had been unwell.

The official news agency KCNA described one of his sons, Kim Jong-un, as the "great successor" whom North Koreans should unite behind.

Pyongyang's neighbours are on alert fearing instability in the poor and isolated nuclear-armed nation.

Fears were compounded by unconfirmed reports from South Korean news agency Yonhap that the North had test-fired a missile off its eastern coast before the announcement of Kim Jong-il's death.

Following news of Mr Kim's death, South Korea put its armed forces on high alert and said the country was on a crisis footing. Japan's government convened a special security meeting.

China - North Korea's closest ally and biggest trading partner - expressed shock at the news of his death and pledged to continue making "active contributions to peace and stability on the Korean peninsula and in this region".

After meeting her Japanese counterpart Koichiro Gemba, US Secretary of State Hillary Clinton said she hoped for a stable and peaceful transition in North Korea.

"We reiterate our hope for improved relations with the people of North Korea and remain deeply concerned about their well-being," she said.

North and South Korea are still technically at war, and the US has nearly 30,000 troops stationed in South Korea. Last year the North was accused of sinking a South Korean patrol boat and the two countries exchanged fire across the disputed maritime border.

Meanwhile, the UN General Assembly overwhelmingly backed a resolution condemning human rights violations in North Korea. The vote, scheduled before Mr Kim's death was announced, called for an end to "systematic, widespread and grave violations". North Korea rejected the resolution.

Asian stock markets fell after news of Mr Kim's death was announced.

Mr Kim's death was announced in an emotional statement on national television.

The announcer, wearing black, struggled to keep back the tears as she said he had died of physical and mental over-work.

KCNA later reported that he had died of a "severe myocardial infarction along with a heart attack" at 08:30 local time on Saturday (23:30 GMT Friday).

He had been on a train at the time, for one of his "field guidance" tours, KCNA said.

The state news agency said a funeral would be held in Pyongyang on 28 December and Kim Jong-un would head the funeral committee. A period of national mourning has been declared from 17 to 29 December.

Images from inside the secretive state showed people in the streets of Pyongyang weeping at the news of his death.

Ruling party members in one North Korean county were shown by state TV banging tables and crying out loud, the AFP news agency reports.

"I can't believe it," a party member named as Kang Tae-Ho was quoted as saying. "How can he go like this? What are we supposed to do?"

Another, Hong Sun-Ok, said: "He tried so hard to make our lives much better and he just left like this."

KCNA said people were "convulsing with pain and despair" at their loss, but would unite behind his successor Kim Jong-un.

It said millions of North Koreans were "engulfed in indescribable sadness".

"All party members, military men and the public should faithfully follow the leadership of comrade Kim Jong-un and protect and further strengthen the unified front of the party, military and the public," the news agency said.

Little is known about Kim Jong-un. He was educated in Switzerland, is aged in his late 20s and is believed to be Kim Jong-il's third son - born to Mr Kim's reportedly favourite wife, the late Ko Yong-hui.

Kim Jong-un was unveiled as his father's likely successor just over a year ago. Many had expected to see this process further consolidated in 2012.

South Korea urged people to "go about their usual economic activities" on Monday, while putting the military on alert.

President Lee Myung-Bak spoke to US President Barack Obama by telephone and they "agreed to closely co-operate and monitor the situation together", a South Korean presidential spokesman said.

The two countries' defence ministers also spoke and "agreed that it is critical to remain prudent with respect to all matters related to our security posture there", a US Defense Department spokesman said.

China said it was "distressed" to hear the news of Mr Kim's death. "We express our grief about this and extend our condolences to the people of North Korea," foreign ministry spokesman Ma Zhaoxu was quoted by Xinhua news agency as saying.

Analysts say that with the process of transition from father to son incomplete, Mr Kim's death could herald "very unstable times" in North Korea.

Kim Jong-il inherited the leadership of North Korea from his father Kim Il-sung.

Shortly after he came to power in 1994, a severe famine caused by ill-judged economic reforms and poor harvests left an estimated two million people dead.

His regime has been harshly criticised for human rights abuses and is internationally isolated because of its pursuit of nuclear weapons.

Under Mr Kim's leadership, funds have been channelled to the military and in 2006 North Korea conducted its first nuclear test. It followed that up with a second one three years later. Multinational talks aimed at disarming North Korea have been deadlocked for months.

He had reportedly been in poor health since suffering a stroke in August 2008.