Sheikh Ali Salman, head of the al-Wefaq Islamic Society, was arrested on Sunday after leading a protest rally against last month’s elections.

Security forces clashed with demonstrators outside the home of the detained head of Bahrain’s main Shi’ite Muslim opposition group on Tuesday, a Reuters journalist said.

The security forces fired tear gas rounds to disperse the gathering in Bilad al-Qadeem village and some protesters hurled rocks and iron rods at police, he said.

Sheikh Ali Salman, head of the al-Wefaq Islamic Society, was arrested on Sunday after leading a protest rally against last month’s elections. He was remanded in custody for a further week on Tuesday, an al-Wefaq official said.

The Bahraini government could not be immediately reached for comment on the clash.

In Geneva, the United Nations human right office said it was concerned at the detention and that a potentially lengthy prison sentence could be handed to Salman.

“Opposition parties are fundamental pillars of any democracy and Sheikh Salman’s arrest risks intensifying the fraught political scene that has seen anti-government protests for nearly four years,” a spokeswoman for U.N. Human Rights chief Zeid Ra’ad al-Hussein Zeid’s office said in a statement.

Salman’s lawyers said he had been accused of serious charges, including calling for the overthrow of the government, the spokeswoman said.

The island kingdom, whose government has close relations with the United States and Britain, has been gripped by tension since 2011 protests led by majority Shi’ite Muslims demanding reforms and a bigger role in running the Sunni-led country.

Bahrain has faced sporadic unrest since authorities quelled the 2011 protests with support from other members of the Gulf Cooperation Council. Demonstrations have increasingly given way to bomb attacks on the security forces. At least two people were killed in two separate attacks this month.

Salman was detained on Sunday after he led a rally last week to protest against a general election which the opposition boycotted.

Al Wefaq had said it would not take part in the poll because parliament would not have enough power and because voting districts favoured the kingdom’s Sunni Muslims.

Al Wefaq media official Taher al-Moussawi said Salman’s detention had been extended for a week.

One of Salman’s lawyers, Abdullah al-Shamlawi, said the prosecution had decided to detain Salman for a week and that questioning would continue on Wednesday.

Bahrain’s attorney general, Nayef Yousef Mahmoud, was cited as saying on state news agency BNA that the public prosecution ordered that the secretary general of a political party be held for seven days pending an investigation. It did not name Salman.

Mahmoud was quoted as saying that the detained man was being questioned about incitement to change the political system by force.