The arrests could heighten tensions with Bahrain's majority before the Oct. 23 election that will be the US-allied island state's third since its king launched a political reform process a decade ago to help quell protests.

Clashes erupted in at least two villages on Saturday night following the Friday arrest of the first of the activists, said Nabeel Rajab, head of the Bahrain Center for Human Rights.

"Three more were arrested this morning," lawyer Mohammed Al-Tajer told Reuters.

Bahrain regularly sees night-time clashes between security forces and protesters in towns.

An Interior Ministry official declined to comment on the arrests, while the prosecutor's office could not be reached.

Tajer said the head of human rights at the Haq movement, Abduljalil Singace, was detained on Friday on his return from London where he gave a lecture on human rights in Bahrain.

Bahrain's state news agency reported that Singace had been detained "based on information affecting national security at home and abroad that damages the country's stability."

Then on Sunday following the protests, Abdulghani Al-Kanjar, who heads a committee of Bahraini human rights groups to support victims of torture, was detained along with cleric and activist Mohammed Al-Magdad.

A third activist, Saeed Al-Nouri, was also arrested. Tajer said it was not clear what the men were being charged with.

Two of the detained men, Singace and Magdad, were among three activists held for several months in 2009 on charges of plotting to overthrow the government. They were later pardoned by the king.

The Oct. 23 elections were not expected to meet opposition demands for more political participation. Many people complain of discrimination in jobs and services, an accusation Manama denies.

Bahrain's largest opposition bloc, Al-Wefaq, plans to participate in the poll where it will field candidates for up to 24 of 40 slots. It currently holds 17 seats it won in 2006.

Bahrain's Parliament is the only one in the Gulf Arab region besides the Kuwaiti Assembly, but its powers are limited as its bills need to be approved by an upper house whose members are appointed by the king.