Bahraini police fired tear gas to break up a march on Saturday by thousands of Shi'ites mourning a man whom activists said was killed in custody but officials said had drowned, residents said.

Dozens of pro-government Sunni militants attacked the mourners, as riot police tried to break up the clashes and keep the two sides apart in Muharraq, a town north of the capital Manama, they said.

Some shop windows were broken during the clashes. I know one was owned by a Shi'ite and one by a Sunni, said a resident adding that police detained at least four people.

Clashes between security forces and mainly Shi'ite opposition activists have taken place on an almost daily basis after the Sunni-dominated government crushed a pro-democracy uprising last year.

The island nation, home to the U.S. Fifth Fleet, is seen by the United States and Saudi Arabia as a key ally against non-Arab Shi'ite power Iran just across Gulf waters.

Police warned mourners several times (the) procession was unauthorized before using legal procedures to disperse the crowd and protect civil peace, the Interior Ministry said on its Twitter page.

The main Shi'ite opposition group Wefaq called the police intervention against the funeral a violation of human and religious rights, according to its Twitter message.

The funeral was for a 24-year-old Shi'ite found dead a few days after he went missing. The Bahrain Centre for Human Rights said his body showed signs of torture, but the Interior Ministry said he suffered from psychological problems and had drowned.

Inspired by Arab uprisings in Tunisia and Egypt, thousands of mainly Shi'ite Bahrainis took to the streets last February and March demanding curbs on the power of the ruling Sunni Muslim Al-Khalifa family and an end to perceived discrimination.

The broader pro-democracy movement was suppressed with military backing from Bahrain's Sunni-led Gulf neighbours Saudi Arabia and the United Arab Emirates.

At least 35 people, including five members of the security forces, were killed in the unrest, according to an inquiry Bahrain commissioned into the protests and their aftermath. The inquiry said it found evidence of systematic abuse and torture.

Bahrain has promised to implement the inquiry's recommendations, which the U.S. Congress has linked to its approval of a $53 million arms sale to Manama. Opposition groups doubt the kingdom's commitment to reform.

(Writing by Firouz Sedarat; editing by Andrew Roche)