American tanks and armoured vehicles have penetrated deep into the centre of the Iraqi capital, raiding President Saddam Hussein's main palace and attacking several other sites.
US troops searched palaces
More than 100 armoured vehicles, including up to 70 tanks, stormed into the Iraqi capital early on Monday, supported by tank-busting A-10 Warthog planes and pilotless drones, US military sources said.
A Pentagon official told the BBC that the US military operation is a "show of force" that sends a powerful message to the Iraqi regime - but is not necessarily the "battle for Baghdad".
As the US military operation continued, Iraqi television broadcast footage of a meeting chaired by Saddam Hussein at an undisclosed location.
Also filmed at the meeting were the president's son Qusay, Deputy Prime Minister Tariq Aziz and Vice-President Taha Yassin Ramadan.
US Central Command has refused to comment on how long troops will stay in the city.
Spokesman General Vincent Brooks said commanders on the ground "will make decisions on what parts of Baghdad they will retain control of".
The BBC's Rageh Omaar in Baghdad says weapons fire during the battle was so continuous it "almost sounds like an ammunition store going off".
He says he heard heavy machine-gun fire and mortars during a fierce battle raging near the presidential palace on the bank of the River Tigris.
The presence of American armour on the edges of the Republican Palace presidential compound is "highly symbolic", our correspondent adds.
US officials said American troops had also secured a second presidential compound in the city.
No figures on Iraqi casualties are available.
Senior officials at US Central Command have urged reporters not to overestimate Monday's attack, saying the operation may not be intended to take and hold ground.
Iraqi Information Minister Mohammed Saeed Sahhaf denies that Baghdad's defenders have lost ground, claiming that Iraqi forces have "slaughtered" columns of US troops.
"Don't believe these invaders and these liars. There are none of their troops in Baghdad," the minister told reporters at an impromptu outdoor press conference.
In other military developments:
- An Iraqi rocket attack on a US command centre south of Baghdad leaves two soldiers and two journalists dead and several other soldiers wounded, according to US military sources at the site
Iraqi troops fled the scene
- At least two US marines are killed and many are injured as they fight to capture bridges in the south-east of Baghdad
- Iraqi soldiers are seen fleeing the scene of battle, shedding uniforms and weapons as they go
- There are conflicting reports about who controls the symbolically significant Al-Rashid Hotel, which has bunkers believed to have been used by Saddam Hussein
- UK troops push into the old city of Basra as they establish control over Iraq's second city
- The northern city of Mosul is subjected to heavy bombardment
There have been reports that the top Iraqi commander in the south of the country has been killed, but UK Defence Secretary Geoff Hoon says he cannot confirm them.
He said his "best judgement" would be that Ali Hassan al-Majid - a cousin of Saddam Hussein known as Chemical Ali - had been killed when his home was bombed at the weekend.
'Chemical Ali': Reported dead
"We have some strong indications that he was killed in the raid conducted Friday night but I can't yet absolutely confirm the fact
that he is dead," he told a news conference in London.
Correspondents say his death would help explain why there has been only sporadic Iraqi resistance in the south of the country.
In Baghdad, a US army colonel interviewed by American television outside the Republican palace said he and his men had been conducting searches of the complex.
Television pictures from inside the palace showed US soldiers amidst ornate furniture covered with grime following the fighting in the city.
The BBC's Andrew Gilligan in Baghdad says relatively small numbers of US troops are involved in the attack.
The US military says its troops have destroyed 60 Iraqi vehicles, three tanks and three armoured personnel carriers in the operation so far. There has been no independent confirmation.
More than 100 US vehicles drove into the heart of Baghdad
The Information Ministry and the Foreign Ministry remain in Iraqi hands, news agencies report.
The attack follows another night of air raids on the city which intensified as dawn broke with bombs striking the centre of the city.
The Associated Press reported that US F-16 fighter jets bombed Iraqi tanks and armoured personnel carriers as US ground forces advanced into Baghdad.
In other developments:
- US President George W Bush and UK Prime Minister Tony Blair are due to discuss the progress of the war when they meet in on Monday in Northern Ireland
- Two Polish journalists are reported missing - a colleague says they were detained by armed Iraqis at a checkpoint in southern Iraq
- US military officials say commander of US-led invasion force, General Tommy Franks, has visited troops at three locations in Iraq
- Speaking ahead of a Security Council meeting, UN Secretary-General Kofi Annan says he expects the UN to play an important role in post-war Iraq.