Al-Sistani's return comes as US and Iraqi forces tightened their siege of Najaf's Imam Ali mosque, where supporters of Shia leader Muqtada al-Sadr are holed up.
"He has entered Iraq through Basra. He will head to Najaf tomorrow," Hamid al-Khafaf, an al-Sistani aide based in London, said on Wednesday.
Al-Khafaf called on Iraqis "to be ready … to march on the city of Najaf under the leadership of al-Sistani to save the city."
Iraq's most senior Shia Muslim figure, Iranian-born al-Sistani has returned from Britain where he had been treated for a heart condition. Al-Khafaf told Aljazeera that the Ayat Allah had overidden doctors' recommendations not to travel.
Aides of al-Sadr, whose al-Mahdi Army militiamen have resisted attempts by US-led forces to expel them from the revered Imam Ali mosque complex, told Aljazeera they welcomed al-Sistani's proposal to lead marchers to Najaf.
"People welcome the return of his eminence Ayat Allah Ali al-Sistani and now men, women and children, in groups and individually, are heading to the city of Najaf to lift the siege imposed by the US occupation forces," said Aws al-Khafaji, an al-Sadr's spokesman from the southern Iraqi city of Nasiriya.
Al-Sadr 's supporters have been
besieged for three weeks
Asked about demands by al-Sistani's aides for Mahdi Army fighters to leave the shrine, al-Khafaji blamed besieging US-led forces for preventing a peaceful withdrawl.
"We repeatedly call on a peaceful solution but ... no one can leave the shrine as US snipers have taken up positions on the roofs of the neighboring buildings while the shells are falling here and there," said al-Khafaji.
"It is better that the fighting ceases so all those conducting their sit-in can leave safely".
'Silent' claim rejected
Al-Khafaf rejected charges that the Iranian-born al-Sistani, who has urged his compatriots not to take up arms against occupation forces, had been curiously silent over the situation in Najaf.
"It is absolutely incorrect. Despite his serious illness, his eminence and the team accompanying him were following the situation in Iraq.
"He has not spared any efforts to end the crisis peacefully. He has proceeded with contacts there that were not reported in the media."
Al-Sistani's propposed march is likely to put al-Sadr's movement under further pressure to withdraw from the mosque, whose occupation by al-Mahdi Army militiamen has directly challenged the authority of US-backed interim Prime Minister Iyyad Allawi.