The field of battle expanded again in Yemen on Tuesday as a cease-fire between government forces and opposition tribesmen in the capital broke down, renewing fears that the country's continuing political stalemate could drag it into civil war.
The fighting came a day after the government pounded a major coastal city with airstrikes to dislodge Islamic militants and, to the west, smashed the country's largest antigovernment demonstration in clashes that killed at least 20 protesters, according to witnesses reached by telephone.
Artillery explosions and machine-gun fire echoed across the center of the capital, Sana, late Monday and Tuesday morning as fierce fighting shattered a tenuous truce that lasted less than two days.
Black smoke rose over the Hasaba neighborhood where security forces attacked a compound belonging to the family of Hamid al-Ahmar, the strongest tribal rival to President Ali Abdullah Saleh, as tribesmen loyal to the Ahmars retook a government building near the compound that they had vacated as part of the truce deal on Sunday.
Violence broke out in Sana a week ago after Mr. Saleh refused to follow through on his promise to sign an agreement leading to his resignation following months of street protests against his rule. It was the third time since the uprising began in January that Mr. Saleh had agreed to transfer power, and the third time he reneged.
The fighting on Tuesday was centered in the same area as last week around the Ahmar compound in Hasaba, where many government ministries are located. Witnesses said a local police station was burned to the ground before dawn on Tuesday. The two sides traded artillery fire near the state-run television headquarters.
"Last night's clashes were the fiercest so far," Mohammed al-Quraiti, a neighborhood resident, told Reuters. "My children and I couldn't sleep all night because of the heavy shooting."
Street battles in the capital reopened a central front for Yemen's security forces, which have moved forcefully to contain a diverse group of distinct opponents, including tribal fighters, militant Islamists and nonviolent antigovernment protesters.
The latter group found themselves the target of a harsh crackdown in the city of Taiz late Sunday and early Monday as security forces and plainclothes gunmen swept through a main square, dispersing the thousands of protesters seeking the ouster of Mr. Saleh. The United Nations High Commissioner for Human Rights said it had received reports that as many as 50 people were killed in the ensuing clashes, The Associated Press reported.
On Tuesday, protest leaders responded to the crackdown with calls for mass demonstrations. A large number of Republican Guard troops deployed around the city to prevent protesters from gathering, witnesses in the city said. News agency photographs from Taiz showed security forces using gunfire to scatter those who tried to protest on Tuesday.
In Zinjibar, along the country's southern coast, residents said warplanes attacked militant positions with repeated bombing runs Monday afternoon, a day after Islamists of uncertain affiliation took control, seizing banks and a central government compound. The compound was hit in the airstrikes and shelled by the army, witnesses said.
It is unclear how many people have died in the Zinjibar fighting, which began Friday. The city has few medical services left, and no electricity or water, people there say. Hundreds have fled; others have taken refuge in local mosques, residents said.
Witnesses said that security forces descended from three directions in a thick cloud of tear gas late Sunday afternoon, and that the clashes continued until after midnight with security forces firing water cannons and setting the protesters' tents ablaze with gasoline bombs. Video posted on social networking sites by opposition groups showed protesters scattering as plainclothes gunmen fired from doorways and from rooftops. Bulldozers and tractors demolished remnants of the sit-in. Sporadic gunfire echoed through the city on Monday, witnesses said.A doctor, Taiz Hamoud Aqlan, said Monday night that he could confirm 20 deaths, but that he expected the number to rise. "I know that there are injured people who we can't even get to because of the constant gunfire," he said. Some reports put the death toll as high as 70, but they could not be confirmed.
The United States Embassy in Sana condemned the "unprovoked and unjustified attack on youth protesters" in a statement, adding that the protesters had "shown both resolve and restraint and have made their viewpoint known through nonviolent means."
A hospital within the protest area was looted on Monday, forcing the wounded to seek assistance farther away, said Abdulkafi Shamsan, a doctor there. He said about 15 soldiers held nurses at gunpoint as they smashed computers, stole medical supplies and detained several patients. "They even shot their guns inside the hospital," he said.
Mohammed Dabwan, a nearby resident, said no protesters had returned to the square on Monday.
Yemen’s state-run media, quoting an unnamed government security official in Taiz, said the violence there was not an organized crackdown. The official said "armed groups" from the opposition coalition attacked a security station, setting fire to cars. The protesters then "kidnapped soldiers and took them to their sit-in square," he said, where they were abused by the protesters. The official said the security forces then "decided on their own to go to the square and liberate their colleagues and clear the square from those making the riots, sabotage and murders."
Witness in Taiz also said the fighting was touched off by a clash at a security station near the protest, but disputed that any soldiers had been kidnapped.
Reporting on events in Yemen was limited Monday by what appeared to be a block on international calls to phones belonging to Sabafone, a cellular network owned by Mr. Ahmar, Mr. Saleh's biggest tribal rival.
Nasser Arrabyee reported from Sana, and J. David Goodman from New York. Laura Kasinof contributed reporting from Hagerstown, Maryland.