BAGHDAD (Reuters) - Explosions rocked Baghdad at dawn on Thursday as the United States launched a war to overthrow Saddam Hussein.

Jets roared overhead, Iraqi anti-aircraft batteries opened up and air raid sirens sounded, said Reuters correspondent Nadim Ladki, reporting from the city centre.

The United States said the attack on Iraq had begun, just 90 minutes after its deadline for the president to quit Iraq had passed.

Bush, accusing Saddam of hiding chemical and biological weapons, set the ultimatum for him to flee into exile or face war with US-led forces.

Ladki said the first blasts seemed to come from the southern and eastern suburbs. Heavy plumes of black smoke billowed from the east after the same target appeared to have been hit three or four times. Several explosions later hit the city centre.

Ladki heard the first two or three blasts at 5:33 a.m. (0233 GMT), an hour and a half after the US deadline for war had passed.

"I clearly heard two to three explosions and warplanes flying over at around medium altitude," Ladki said in his first report. He heard a further four explosions, apparently in the city centre, some minutes later.

A few cars moved hurriedly on otherwise deserted streets. Air raid sirens began wailing out within a minute of the first strike. Electricity was still working in the city centre.

Iraqi radio went off air after the explosions - and appeared to have been taken over by the US military, broadcasting in Arabic on the same frequency.

There was no sign the Iraqi president had complied with Washington's demands when Bush's 48-hour ultimatum ended at 4 a.m. (0100 GMT).

The Iraqi capital had taken on the air of a ghost town on the eve of the deadline. Shops and restaurants were shut and the streets deserted.

Trenches have been dug and sandbags placed around official buildings and handfuls of militiamen with Kalashnikov rifles posted outside in recent days.

To the south, where US and British forces are massed on the ground in Kuwait and aboard ships in the Gulf, there was little sign of an increase in activity overnight on the ground.

Reuters correspondent David Fox, in northern Kuwait close to the Iraqi border, said there had been little movement among the troops, who number more than 280,000 in the region as a whole.

Some combat units had moved into the demilitarised zone that straddles the Iraq-Kuwait border on Wednesday.

Bush and British Prime Minister Tony Blair have massed 280,000 troops in the region to kill or capture Saddam, overthrow his government and rid Iraq of chemical and biological weapons the United States accuses it of stockpiling.

Iraq denies having such weapons.

The zone, monitored by the United Nations since the 1991 Gulf War, extends three miles into Kuwait and seven miles into Iraq. Soldiers donned chemical suits at desert staging posts that were swept by fierce sandstorms.

US officials had said upward of 3,000 satellite-guided bombs and cruise missiles would be unleashed from sea and air on targets vital to Saddam's government to start the war.

In northern Iraq, where Kurds run their own affairs under the protection of another post-Gulf War no-fly zone, the de facto frontier with Saddam's forces was also quiet.

Reuters correspondent David Hemming, on the Kurdish side of the border near the town of Dohuk, said Kurdish fighters told him ten Iraqi military trucks had moved up to the Iraqi lines on Wednesday evening and local residents said Kurdish "peshmerga" guerrillas had reinforced their own lines.

Hemming also said a handful of European-looking men in civilian clothes had been heading for a position nearby with Kurdish guards. Kurdish fighters say they have been cooperating with US special forces in the region, which lies close to the Iraqi cities of Mosul and Kirkuk, the main oil centre in the north.

The United States had wanted to open a northern front to attack Iraq from NATO ally Turkey.

But, faced with popular opposition to war on fellow Muslims, Ankara balked at letting U.S. troops onto its soil.