It was the ninth straight day of protests in Yemen inspired by uprisings in Egypt and Tunisia. Demonstrators are calling for the ouster of President Ali Abdullah Saleh - a key U.S. ally in fighting al-Qaida terrorists - who has ruled the country for 32 years.
U.S. President Barack Obama condemned the reported violence in response to protests in Yemen, as well as Bahrain and Libya, he urged those governments to respect the rights of peacefully demonstrating citizens, and he expressed condolences to the families of those killed.
The U.S. Embassy in Sanaa said it has seen "a disturbing rise in the number and violence of attacks against Yemeni citizens" at peaceful protests. It added that diplomats also saw reports of Yemen government officials "present during these attacks," which it called "contrary to the commitments that President Saleh has made to protect the right of Yemeni citizens to gather peacefully to express their views."
Saleh is already facing a restless population, with threats from al-Qaida militants who want to oust him, a southern secessionist movement and a sporadic armed rebellion in the north.
To try to quell the new outbursts of dissent, Saleh pledged to meet some of the protesters' demands and has reached out to tribal chiefs, who are a major base of support for him. But a key chief from Saleh's own tribe was critical of his policies and threatened to join the protesters - an apparent attempt to pressure the embattled leader of the world's poorest Arab country.
For now, most of the protesters are students, educated professionals and activists who used social media sites Facebook and Twitter in summoning people to the streets for the "Friday of Rage" following noon prayers. Tens of thousands responded in the capital of Sanaa, the southern port of Aden and the political hotbed of Taiz. Some websites also referred to the day as "Friday of the Beginning."
In a change, many mosque preachers took a critical tone toward the government.
A preacher at the Sanaa University mosque spoke out against violence against demonstrators, telling many protesters who had gathered there: "We have been living for 30 years without purpose or hope."
Another Sanaa preacher, Imam Abdel Raqib Obad, urged people to join the protests and criticized security forces for "battling" youths.
Imam Jabri Al Yamani admonished the crowd that "protests must be peaceful and not scare and harm the people," but as demonstrators marched toward the presidential palace afterward, the scene descended into violence.
The crowd, chanting anti-government slogans, was met by a heavy deployment of riot police and hundreds of Saleh supporters, similar to confrontations earlier this week. The pro-and anti-government sides attacked each other with rocks, and the riot police began firing in the air and launching tear gas canisters.
At least four people were hurt seriously enough to be taken away by ambulances.
The demonstrators dispersed to other streets, some of which were blocked by police.
Journalists also came under attack by government supporters. An Associated Press reporter saw men with sticks attack a TV crew, smashing their camera. Other photographers took refuge in a building to avoid the mob.
In Taiz, about 270 miles (435 kilometers) south of Sanaa, the call for demonstrations brought out thousands of people, and witnesses said men in a speeding civilian car threw what appeared to be a grenade into a crowd of demonstrators, causing a stampede when it exploded.
At least 48 protesters were seriously wounded, and many others were hurt as they fell in the stampede and chaos, according to medical officials and Ghazi al-Samie, a lawyer and activist in Taiz, Yemen's second-largest city.
In Aden, protesters burned four government cars and a local council building. The clashes continued well after sundown, when protesters set a police storage area ablaze and burned tires.
The police fired into the crowd, using tear gas, rubber bullets and what some witnesses said was live ammunition. Four protesters were killed - including two in their 20s who were shot, according to witnesses and medical officials who spoke on condition of anonymity because they were not authorized to talk to the press.
At a meeting between Aden officials and Vice President Abd-Rabo Mansour Hadi, officials called for the resignation of the police chief, whom they held responsible for the violence, according to an official at the session speaking on condition of anonymity because he was not authorized to discuss the details.
A large demonstration also was reported in eastern Hadramawt province, where police fired in the air to disperse protesters.
The violence forced international soccer officials to postpone Yemen's Feb. 23 home qualifying match for the 2012 Olympics against Singapore. The match, in Sanaa, was put off until March 2.
Saleh, a weak but important partner for Washington, had pledged not to seek re-election in 2013 or pass power to his son. The promise was seen as an attempt to defuse calls for his ouster.
Opposition groups said they are suspicious of Saleh's offer and want concrete proposals for change.
Tribal chief Hussein al-Ahmar, a member of Saleh's tribe, told a crowd in his home province Thursday that if violence against protesters continued, his tribesmen may come in to the defense of the demonstrators.
The threat, apparently aimed to get more concessions from Saleh, could turn the protests more violent. Al-Ahmar's tribe, Hashed, is one of the largest and best-armed in Yemen.
"It is not the armed forces, nor Saleh and his army that protect Sanaa, but the tribes of Hashed" and others, Al-Ahmar told the crowd in Amran province. "If the authorities continue to scare the protesters with their thugs, we will have to interfere."
Yemen has become a main battleground against al-Qaida. The government, which receives millions of dollars in U.S. military aid, has allowed American drone strikes on al-Qaida targets and has stepped up counterterrorism cooperation.
The U.S.-born radical cleric Anwar al-Awlaki, thought to be hiding in Yemen, is believed to have inspired and even plotted or helped coordinate recent attacks on the U.S. Those include the failed December 2009 bombing of a Detroit-bound airliner and the unsuccessful plot to send mail bombs on planes from Yemen to the U.S. Al-Awlaki also is believed to have inspired the 2009 shooting at Fort Hood, Texas, and had ties to some of the 9/11 hijackers.
Nearly half of Yemen's population lives below the poverty line of $2 a day and its government is riddled with corruption. The country also is plagued by shrinking water and oil resources and an inability to feed its people. Poverty and malnutrition are rampant in the country's rugged hinterlands.