An opposition activist was shot dead in a gunfight with pro-regime loyalists in north Yemen on Tuesday, a security official said.
Naser Musleh Nasm, a member of the main Islamist opposition, was "killed in the armed clash with supporters of the ruling General People's Congress (GPC)" in Al-Jawf province, a security official told AFP and tribal leaders confirmed.
The same sources said that security forces and GPC supporters are surrounding a government headquarters which the protesters have controlled since Monday.
"The police have warned the demonstrators that they should evacuate the building or else they will be evicted by force," a tribal leader said.
On Monday, 20 people demanding the resignation of President Ali Abdullah Saleh, in power for 32 years, were injured when they assaulted the government headquarters in Al-Jawf, northeast of the capital Sanaa, an official said.
Security forces and pro-regime loyalists, who were guarding the building and opened fire in an effort to break up the protest, were wounded by stones hurled by the protesters.
Al-Jawf, a stronghold of the Zaidi rebels, also known as Huthis, on February 22 joined anti-Saleh protests which erupted across the poverty-striken country in January and gained momentum last month.
Yemen's mountainous northern region is stronghold of the Zaidi rebel movement, which from 2004 fought six wars with Saleh's government forces before signing a truce in February 2010.
Protests against Saleh's regime, inspired by similar popular uprisings that have ousted leaders in Egypt and Tunisia, have been marred by violence that has left dozens dead.
Also on Tuesday, tribal fighters sabotaged an oil pipeline in the eastern Yemeni province of Marib and cut the road between the region's gas fields and the capital, security officials and tribal sources said.
Yemen's tribes have targetted oil pipelines in the past to protest against central government policy.
Yemen, which is also battling secessionist unrest and a Shiite sectarian rebellion, is the poorest country on the Arab peninsula. It is not a member of OPEC but produces some 300,000 barrels of crude oil per day.