Moqtada Sadr, the radical Iraqi Shia cleric, has appeared in public in Iraq for the first time in months.
Moqtada Sadr delivered a fiery anti-American sermon in Kufa
He led Friday prayers in Kufa to deliver a defiant sermon condemning the US and the occupation of the country.
A senior aide to Mr Sadr told the BBC that he had left Iraq over fears for his safety and made a regional tour, including a trip to Iran.
Meanwhile, British sources say the leader of the cleric's Mehdi Army in Basra has been killed by Iraqi forces.
However, a source close to the Mehdi militia in the city accused the British Army of carrying out the killing - this is also how Arab news channels are reporting the death.
Abu Qadir, also know as Wissam Waili, 23, was wanted for weapons trafficking, theft, financing illegal militias and carrying out attacks on British forces, the British Army says.
A spokesman said Iraqi forces had tried to arrest him and that the militia leader had died in an exchange of fire. He denied British troops were involved.
Moqtada Sadr, in a characteristically fiery anti-American sermon delivered in Kufa, demanded that US forces leave Iraq.
He led the 6,000 worshippers in the mosque in chanting: "No, no for Satan. No, no for America. No, no for the occupation. No, no for Israel."
However, the cleric urged his followers to use peaceful means of opposition.
The Mehdi army has fought serious rebellions against US forces
His Mehdi Army, a Shia militia responsible for some of the sectarian killings in Iraq, has become one of the targets of the US-led surge.
During his absence, Mr Sadr withdrew six ministers loyal to him from the Iraqi cabinet in an effort to press Prime Minister Nouri Maliki to set a timetable for a US troop withdrawal.
In 2004, the Mehdi army fought two serious rebellions against US forces but when the US began its security drive in Baghdad in February, Mr Sadr ordered his militants off the streets to avoid confrontation.
The cleric's brand of nationalism and populism has made him a popular figure among Iraq's Shia Muslims, but it is not clear why he has chosen this moment to appear in public again.
In a BBC interview, the chairman of Mr Sadr's political committee confirmed the young cleric had left Iraq because he was "personally targeted".
He said Mr Sadr had a political agenda - as yet undeclared - which included "correcting the path of the current government".
He said political pressure needed to be applied to the occupation forces "in order to give this government more security responsibilities and more control over the country".
'Next months vital'
Moqtada Sadr is one of the most important players in Iraq's complex sectarian and political mosaic, says the BBC's security correspondent Rob Watson.
One theory for his return is a desire to reassert control over his militia, which is reported to be increasingly fragmented.
Mr Sadr may also see a chance to strengthen his position in the absence of his great Shia rival Abdul Aziz Hakim, who has left Iraq for medical treatment, our correspondent says.
President Bush warned of more heavy fighting in Iraq
One senior US official described Mr Sadr as a highly unstable 33-year-old whose own aides often find hard to predict.
If he calls his militia back onto the streets of Baghdad they will run into the thousands of extra US troops deployed there in an effort to curtail sectarian killings.
Gen Peter Pace, the chairman of the US joint chiefs of staff, said the number of sectarian murders had risen in May but was still well below January's level, before US troops began to be deployed.
On Thursday, President Bush said the next few months would be vital to the new US security strategy in Iraq and he warned that more heavy fighting could be expected.
He said the last of the 30,000 US troop reinforcements would arrive in Baghdad by the middle of June.
The death toll for American soldiers in Iraq this month - about 90 - is one of the highest since the invasion in March 2003. About 3,440 have been killed since then.