Armed men have attacked a prison in Bahrain where inmates convicted over anti-government protests are held, killing one policeman and allowing an unknown number prisoners to escape.

Security forces launched a manhunt for the attackers and the escapees, the interior ministry said on Twitter on Sunday.

The attack took place at Jau prison south of the capital Manama.

"An armed attack on the Jau Reform and Rehabilitation Centre resulted in the death of policeman Abdul Salam Saif, and the escape of a number of inmates convicted of 'terrorist' crimes," the ministry said.

A Bahraini court in January last year sentenced 57 men to 15-year jail terms for taking part in a 2015 riot inside Jau prison.

Bahraini security forces tear-gassed and beat inmates at the prison while trying to quell clashes that erupted during family visits, local human rights group Bahrain Youth Society for Human Rights said at the time.

In June, 17 prisoners broke out of Al-Hadd jail, east of Manama, but the authorities recaptured 11 of them the next day.

Bahrain has been rocked by unrest since its Sunni authorities crushed Shia-led protests in 2011 demanding a constitutional monarchy and an elected prime minister.

Hundreds of Shia have been arrested and many have faced trials over their role in the demonstrations.

Shia protesters continue to frequently clash with security forces in areas outside Manama.

The Sunni-led government blames Al-Wefaq, the country's largest opposition movement, for the upheaval, accusing the Shia party of pursuing an "Iranian agenda".

Al-Wefaq, for its part, denies the allegations, saying it wants to see the establishment of a "constitutional government" in the small Gulf kingdom.