Moqtada Sadr, the radical Shia cleric, has backed a peace plan with Sunni factions in a bid to calm Iraq's sectarian violence.
Moqtada Sadr's faction has met for talks with Sunni moderates
Mr Sadr, who appeared in Iraq for the first time in months at Friday prayers, said his followers would co-operate with Sunnis against US occupation.
A senior aide told the BBC Mr Sadr had met moderate Sunni groups, aiming to forge a "united and democratic Iraq".
Iraq's vice-president called Mr Sadr's statement "quite encouraging".
Tarek al-Hashemi described Mr Sadr as "number one... the most influential leader" and said he would welcome a new approach to Sunni-Shia relations.
Speaking in the city of Kufa, Mr Sadr blamed foreign troops for Iraq's problems, and said Sunnis and Shias alike should oppose their continued presence in the country.
"I say to our Sunni brothers in Iraq that we are brothers and the occupier divided us in order to weaken the Iraqi people," he said.
"In unity is strength, and in division weakness. We say to them, welcome at any time.
"I am ready to cooperate with them at all levels. This is my hand I stretch out to them - in so doing, I seek only God's satisfaction."
A senior political aide to Mr Sadr, Abd al-Mahdi al-Mutairi, told the BBC how the cleric's organisation was now seeking a compromise with moderate Sunnis.
He said Sadrist representatives met a group called the Anbar Awakening Council with the aim of preventing "sectarian sedition".
"We want a united and democratic Iraq that does not follow the occupation's agenda," Mr Mutairi said.
The Mehdi army has fought serious rebellions against US forces
"We signed with them a pledge charter which we hope will be the nucleus of future agreements with other brothers, whether Sunni, Kurdish or otherwise."
Mr Sadr's followers have not always preached peace and co-operation.
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.
But when the US began its security drive in Baghdad in February, Mr Sadr ordered his militants off the streets to avoid confrontation.
And during his recent absence from Iraq on security grounds, 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 a characteristically fiery sermon in Kufa, Mr Sadr 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 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 return.
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 re-assert 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.
One senior US official described Mr Sadr as a highly unstable 33-year-old whose own aides often find hard to predict.