His son, Prince Abdullah, was sworn in as his successor during an emergency session of Jordan's two houses of parliament.

Thousands of people gathered outside the hospital where King Hussein died, weeping and chanting his name.

Grief is spreading through Jordan

In Amman, mosque loudspeakers sounded Koranic verses in a sign of mourning. State-run Jordanian TV and radio carried similar verses.

The king died at 11:50am (0950 GMT) at the King Hussein Medical Centre, a senior government official said.

He had been on a life support machine after his internal organs failed.

Jeremy Bowen in Amman: "End of an era"

After months of treatment for lymph cancer, the cause of death was heart failure, said senior palace officials who were with the king when his heart stopped beating.

The 63-year-old King - the Middle East's longest-serving leader and one of the region's great peace-makers - had returned home to die on Friday when doctors in the United States said they could do no more for him.

Political implications

World Affairs Correspondent John Simpson: "Genuine sadness"

The regional implications are likely to be profound.

Israel will be particularly nervous about the death of its strongest ally in the Middle East and Prime Minister Binyamin Netanyahu was one of the first to pay tribute to the king.

Prince Abdullah, who was named as King Hussein's successor just two weeks ago, has said he will continue with his father's policies.

He was officially sworn in as the new king three hours after his father's death was announced.

In front of the parliament he said: "I swear by God that I will respect the constitution of the nation."

The new King Abdullah addresses the people (with translation)

The prince had earlier appeared on Jordanian state television to address the nation.

Prince Abdullah had been made Regent on Saturday after the Jordanian parliament agreed the king was too ill to fulfill his duties.

World tributes

Leaders from around the world paid tribute to King Hussein's vision of Middle East peace.

US President Bill Clinton said: "He won the respect and admiration of the entire world and so did his beloved Jordan.

"A man who believed that we are all God's children, bound to live together in mutual respect and tolerance. King Hussein was ennobled.

UK Prime Minister Tony Blair said: "He was an extraordinary and immensely charismatic persuader for peace.

"At the peace talks in America when he was extremely ill, he was there, talking to both sides, urging them forward, telling them nothing must stand in the way of peace."

Israeli President Ezer Weizman called the king "a brave soldier who fought for peace".

Across the Middle East, Arab nations announced periods of mourning to remember the king.

A statement from the Palestinian Authority said: "President Yasser Arafat and the Palestinian people and leadership have received with great sorrow and pain the news."

National grief

Crowds had been keeping a vigil outside King Hussein's clinic - praying for a miracle while waiting for the inevitable end.

Many more joined them as news of King Hussein's death spread.

King Hussein's subjects, many of whom have never lived under any other monarch, draped pictures of the king in black and others openly wept on the capital's streets.

Jordan has announced 40 days of mourning for the king, whose funeral is to take place on Monday at the Hashemite burial ground in Amman.

About 40 heads of state are expected to attend.

The Israeli and UK prime ministers are among those who have confirmed they will attend the funeral.

Sad echo of triumphant return

King Hussein was suffering from non-Hodgkin's lymphoma, a form of cancer which has a recovery rate of approximately 50%.

Diplomatic Correspondent Barnaby Mason: A king remembered

His wife, Queen Noor and other relatives were at his side when he died.

Just two weeks ago the king made a triumphant return to Jordan, after six months of treatment in the US. He was greeted by hundreds of thousands of people lining the route to the palace. At the time, he declared himself "completely cured".

Just six days later he was rushed back to the Mayo Clinic in Minnesota, after a relapse.

He was given another session of chemotherapy and a bone marrow transplant.

His condition was stable until Wednesday, when it declined severely and King Hussein returned home for the last time.