SANAA, Yemen -- Protesters seeking the end of Yemeni President Ali Abdullah Saleh's 33-year rule clashed with security forces in several cities Monday. Witnesses and medical sources reported at least 12 deaths and hundreds of injuries in the cities of Taiz and Hodeida as security forces turned bullets and tear gas on protesters pressing toward key government buildings.
More than 20 tanks and armored vehicles surrounded the principal government building in Taiz on Monday as security forces tried to hold back advancing protesters, a senior government official said.
Medical sources said 12 people were killed and more than 500 injured Monday as about 90,000 people demonstrated.
At least two of the dead were shot in the chest, according to medical field teams, who said the gunfire came from central security forces and the Republican Guard.
Protests also took place Monday in the capital city, Sanaa, as well ad Aden and Ibb.
Eyewitnesses in Hodeida said snipers in civilian clothing were shooting into a crowd of protesters in Change Square, and security forces were using tear gas on demonstrators.
Witnesses in Hodeida said tens of victims were laid out on the streets awaiting emergency aid.
"The field hospital cannot give health care to all the injured and is coordinating with hospitals outside Taiz to help the injured," said Sadiq Shugaa, the head of the medical team in Taiz.
More than 300 people in Hodeida were suffering from gas inhalation, according to witnesses.
A senior government official in Hodeida said the violence began when protesters attempted to get into the Republican Palace. The official said security forces were only defending government property from theft.
In Taiz on Sunday, three people were killed and 1,700 injured, according to medical sources, who said the casualties were caused by central security police and riot police.
On Sunday, Taiz Gov. Hamoud al-Soufi had denied reports of a killing and said that the clashes did not occur in a city square, but on the main street, the state-run Saba news agency reported. As riot police intervened to clear the road, "infiltrators and some young hotheads" threw stones at soldiers, wounding eight, including one seriously, the governor said.
The same day, Yemen's parliament speaker rejected a transition plan by the country's largest opposition bloc, indicating a continued stalemate over how Saleh should hand over power.
Calls for Saleh's ouster have grown louder in recent weeks following revolutions in Egypt and Tunisia.
Saleh has ruled since 1978 and has been fighting to hold on to power, arguing that he is best equipped to lead the fight against radical Islamists. He has been a staunch U.S. ally in the fight against al Qaeda in the Arabian Peninsula.
Saleh has offered to step down by the end of the year, after constitutional reforms and new elections. The Joint Meeting Parties bloc demands Saleh's immediate ouster, and a plan unveiled Saturday called for Saleh to hand over all authority to Vice President Abdu Rabu Mansour Hadi, a spokesman for the bloc told reporters.
Once power is handed to Hadi, he should change the structure of the security forces -- including the Republican Guard -- in a way that is "fair" and in accordance with Yemen's constitution, the spokesman said.
The bloc wants Hadi, as president, to form a council that focuses on transparency in the military, a national transition council that will represent all factions in the country and a committee to oversee new elections. The bloc also wants him to affirm the right of peaceful protests and investigate claims of brutality against opposition demonstrators.
Speaker of Parliament Yahya Al-Raee, who also is a senior officer of the ruling party, dismissed the bloc's plan, saying "it was prepared during a khat chew and has no value." (Many Yemenis chew khat, a tropical plant that acts as a stimulant, as part of social and business transactions).
In Taiz, eyewitnesses said security police aided by riot police moved in before dawn Sunday to try to disperse demonstrators who had gathered on Saturday. According to the eyewitnesses, some protesters were beaten, and when others tried to help them, the security police fired tear gas.
On the other hand, Tahrir Square, also in Sanaa, belonged to the president's defenders. Thousands have gathered there to show support for Saleh.
The president has said he accepts opposition demands for constitutional reforms and holding parliamentary elections by the end of the year.
But Saleh said as recently as last week that he will not offer any more concessions. Saleh described the opposition as an alliance against the country's majority, according to Saba.
Journalist Hakim Almasmari and CNN's Mohammed Jamjoom and Christine Theodorou contributed to this report.