One activist was killed as government forces shelled Yemen's protest hotbed, Taiz, for a third straight day on Saturday, residents said, despite a call by the country's acting ruler to end the fighting.
The death raised to at least 16 the number of people killed in three days of fighting between troops loyal to outgoing President Ali Abdullah Saleh and opposition fighters.
Saleh handed over power last month to Vice-President Abd-Rabbu Mansour Hadi, seeking to defuse the opposition to his rule, but the violence gave no sign of letting up.
Residents said government forces were using artillery and tank shells and rocket fire in residential areas on Saturday in western Taiz, trapping some 3,000 families in the commercial hub, some 200 km (120 miles) south of the capital Sanaa.
Gunmen loyal to the opposition were responding with medium and light fire, they said.
Tens of thousands of residents defied the shelling and marched in the city centre demanding that Saleh be put on trial. Medics said one of the demonstrators was shot and killed by a sniper during the demonstration.
An official from Saleh's party said the new violence flared up amid differences with opposition parties over the composition of a military committee agreed upon as part of the transition deal signed last month in Saudi Arabia to end 10 months of protests that had paralysed Yemen.
Ten people died in fighting on Thursday, including five government soldiers, and five more were killed on Friday, medics and security sources have reported.
A rights centre run by the opposition put the death toll at 21 people.
State news agency Saba quoted security sources as saying that armed groups were behind Friday's attacks on government facilities, including the criminal investigations building, the regional branch of the Immigration and Passports Department and a military camp.
Prime Minister-designate Mohammed Basindwa, an opposition leader, has warned that his side would rethink its commitments under the transition accord if the fighting in Taiz, in southern Yemen, did not cease.
In a statement, Basindwa said the bombardment was an intentional act to wreck the agreement that opposition parties signed along with Saleh, who had backed out of signing the deal brokered by Yemen's Gulf neighbours three times.
Saba said British Foreign Secretary William Hague told Hadi in a telephone call on Friday that the UK was monitoring the situation in Yemen, hoping that the government of national reconciliation, after it is formed, would ensure security and stability in the capital and in the capitals of the provinces.
Under the agreement the military committee, headed by Hadi, must run the armed forces in the interim period, oversee the end of fighting and return of forces to barracks. The committee is to be made up of an equal number of people from Saleh's General People's Congress (GPC) and the opposition Joint Meeting Parties (JMP).
An official with the GPC said there was disagreement over the seven names nominated by the opposition.
The Saba news agency said Hadi called on the provincial governor in Taiz and opposition parties to agree a truce.
A spokesman for the JMP called for the transition process to be speeded up. What is required is to speed up the establishment of a military committee that will oversee an end to fighting and ensure security ahead of restructuring of the army, said Mohammed Qahtan.
In separate violence, gunmen killed a colonel in the intelligence service at a coffee shop in the city of Ghayl Ba Wazir in Hadramout province, in southern Yemen, on Friday evening, local officials said. A resident was also killed and three were wounded in the attack, blamed on al Qaeda.
It was the latest in a series of attacks in recent months on senior officers in the army and security forces in southern Yemen.
(Reporting by Mohammed Ghobari in Sanaa, additional reporting by Mohammed Mokhashef in Aden, writing by Sami Aboudi; Editing by Alessandra Rizzo)