Clashes between supporters of an outspoken Shia religious leader and Iraqi police have killed at least 25 people in the Shia holy city of Karbala, police sources say.

The violence started late on Tuesday night when supporters of Mahmud al-Hasani al-Sarakhi were prevented from marching on the Imam Hussein shrine by Iraqi security forces, who were supported by helicopters.

Al Jazeera's Imran Khan, reporting from Baghdad, said Sarakhi had criticised other religious leaders in the city for being under Iranian influence.

"His supporters have had enough of Sistani," Khan said, referring to the most revered religious figure in Iraq, Grand Ayatollah Ali al-Sistani, who recently called on Iraqis to support the government in its fight against Sunni rebels.

Sarakhi is reported to be hiding with some of his supporters in his office, which is beseiged by Iraqi forces.

The fighting was the "most intense" Karbala had experienced in recent years, Khan said.