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King Hussein's generals ordered tanks into the capital Amman at dawn using artillery and rockets against mortar fire.
The strategic town of Zerka, that controls supply routes to the north of the country, was also the scene of heavy fighting.
There have been claims and counterclaims of victory. Amman Radio said the Jordanian army controlled three-quarters of the capital while Palestinian sources said they controlled the whole city, where Palestinians make up 70% of the population.
Amman airport and the country's borders are closed, and telecommunications lines are down.
New military government
King Hussein's troops are reported to have stormed the headquarters of the Palestine Liberation Organisation.
But the whereabouts of PLO leader Yasser Arafat are as yet unknown, although he is believed to be in Syria which has supported the Palestinian cause.
Following the 1967 war with Israel, Jordan lost the West Bank of the Jordan River. Thousands of Palestinians fled into Jordan, swelling the refugee population to two million.
From their new base, the PLO launched military operations against Israel and drew bloody reprisals that killed and injured Jordanians.
The recent multiple hijacking of western airliners forced to land in Jordan by Palestinian militants has made the king even more determined to crack down on the guerrillas.
King Hussein condemned the hijackers as "the shame of the Arab world" in an interview with French newspaper Le Figaro earlier this week.
He also warned that if the PLO guerrillas did not respect recent ceasefire agreements "they would suffer the consequences".
He added: "Every day Jordan sinks a little more. There must be peace - or war."
Yesterday he formed a new government of military hardliners led by Brigadier Mohammed Daoud in a move described by Mr Arafat as a "fascist military coup".
For their part, Palestinians are angered by King Hussein's recent involvement in Middle East peace moves with Israel initiated by the United States.
The US Defence Secretary, Melvin Laird, has said that if necessary all 300 US citizens in Jordan will be airlifted out.
He also suggested America may provide military support to King Hussein's government if the situation worsened and said some units of the Sixth Fleet had moved closer to the area in the last 24 hours.
During what became known as Black September, Yasser Arafat ordered the overthrow of King Hussein's "Fascist government".
Syria, Iraq and Israel quickly became involved and Syrian troops invaded Jordan.
The US 6th Fleet moved into the Mediterranean, the Soviet Union began leaning heavily on its ally Syria to pull out and the PLO guerrillas were gradually driven out of the suburbs of Amman.
Yasser Arafat agreed to a ceasefire on 25 September. Under the terms of the agreement signed two days later in Cairo his troops were to withdraw from Jordanian towns and cities and recognise the king's authority.
Fighting continued into 1971 when King Hussein drove out the Palestinians from their remaining bases and expelled them from the country.
The Palestinian extremist group Black September was named after the month in which the Palestinians were driven out of Jordan.
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