British troops have bombarded the headquarters of Iraq's Baath party as the massive coalition assault moves into the centre of Basra.
Marines of 40 Commando advance into the city
The thrust into the southern Iraq city on Sunday will deliver the "liberation of Basra", British forces say.
Television reports put troops from the Royal Scots Dragoon Guards and the Black Watch at the heart of Iraq's second city.
In the south west of the city, Royal Marine Commandos spearheaded a second wave of attacks on Sunday afternoon.
The elite forces began their arm of the pincer movement with heavy
shelling at about 1530 local time (1330 BST).
British Challenger 2 caught up in regular Basra traffic
"This is the liberation of Basra we've been planning for," British forces spokesman Group Captain Al Lockwood told BBC News Online.
The first major assault began at dawn.
The 7th Armoured Brigade, the Desert Rats, stormed the city with several thousand troops and hundreds of tanks in a bid to take control.
Three units, the Royal Scots Dragoon Guards, Royal Fusiliers and the Black Watch, advanced into the city, meeting only "isolated pockets" of resistance, correspondents say. The airport has been secured.
The dramatic push follows days of caution while the mood of Iraq's second biggest city and its defences were assessed.
In other military developments:
A fierce battle has been going on in the western outskirts of Baghdad, says the BBC's Gavin Hewitt who is travelling with US forces. He says the Iraqis were putting up fierce resistance, but he also saw more than a dozen burned out Iraqi tanks and APCs and reports that civilian vehicles also got caught up in the fighting.
- Kurdish and American forces in northern Iraq suffer a setback as a convoy is bombed by US aircraft in another "friendly fire" incident; the BBC's John Simpson, at the scene, reports several dead.
- US forces have begun to airlift Iraqi opposition fighters - under the control of the Iraqi National Congress and its leader Ahmed Chalabi - into southern Iraq, American network ABC reports.
It remains unclear how many Iraqi militia remain in Basra.
British troops say they are being welcomed by civilians waving, cheering and sounding the horns of their cars.
But Iraqis wearing civilian clothes have also thrown a bomb into a building where marines were taking cover.
Captain Roger McMillan of the Royal Scots Dragoon Guards told BBC News no casualties had been suffered and added this was a vindication of the British policy of "advancing slowly and softly as opposed to hitting the door from day one".
The situation was becoming "increasingly benign", he added.
"We have pushed on the door of Basra and it opened.
"We are going to be staying."
Heavy fighting erupted as the sun rose at 0200 GMT.
British tanks and American Cobra helicopter gunships repeatedly pounded a factory complex as Iraqi militiamen responded with machinegun and sniper fire.
A para inspects the quarters of an Iraqi army mortar team
While the fighting raged, looters descended on the building to steal whatever they could lay their hands on.
Then, just after dawn, RAF Chinook helicopters flew in reinforcements of Royal Marines.
Group Capt Lockwood said the resistance encountered had been "generally disorganised, with no real command and
And it appeared
Basra's Baath party leadership had been eliminated or fled, he added.
HUMAN COST OF WAR
US: 71 dead (including 16 in non-combat accidents, 4 to 'friendly fire', 2 under investigation), 7 missing
UK: 27 dead (including 16 in non-combat accidents, 5 to 'friendly fire')
Iraq: At least 1200 civilian deaths, military deaths unknown
*Figures from each government
A suspected suicide bomber handed himself over to British troops in the suburbs the city.
The man, who was in his late 40s or early 50s, walked up to gates of a Royal
Marines' base carrying five hand grenades.
He claimed to have been behind at least two attacks on the base in recent days
but had had enough, an officer said.
One of several people leaving Basra told BBC News there had been very little resistance to the British advance.
"A lot of fighters have run away.
"There was a little bit of shell fire - but now it is quiet."
The troops have been ordered not to destroy or desecrate Iraqi flags or war memorials.
Only symbols of the Iraqi regime are deemed justifiable targets.
On Sunday US Central Command spokesman Jim Wilkinson said the bodies of bodyguards had been found after British forces attacked a Baath party building in which Ali Hassan al-Majid,was believed to be hiding.
A would-be suicide bomber hands himself in to Royal Marines
The Iraqi commander, known as "Chemical Ali", is in charge of the south of the country
The BBC's Caroline Wyatt, with the British troops, also said sources had suggested there was "good reason" to think he may have been killed, "and that would be very, very significant".
Thousands of people reportedly cheered when the building was destroyed.
In other developments:
- A Russian diplomatic convoy evacuating staff from Baghdad is attacked, with several people reportedly injured.
- US deputy defence secretary Paul Wolfowitz says it will take more than six months for an Iraqi government to be created to run the country after the defeat of Saddam Hussein's regime.
An Iranian general says that hundreds of bodies discovered in a makeshift morgue by British forces near Basra are those of Iranian soldiers killed in the 1980s Iran-Iraq war
- American officials say the US is to start moving its planned post-war civil administration, led by retired general Jay Garner, into Iraq within the next few days.
- Sheikh Muhammad Sayyed Tantawi, the grand imam of Egypt's al-Azhar Mosque - seen as the highest spiritual authority for Sunni Muslims - says Saddam Hussein was wrong not to go into exile to avert the US-led war.