(CNN) -- Yemeni President Ali Abdullah Saleh returned after undergoing treatment in Saudi Arabia, a government spokesman said Friday, and was greeted by a nation in turmoil.
Saleh returned to Yemen after a three-month medical stay in Saudi Arabia, said Mohammed Albasha, the Yemeni government spokesman.
Protesters have been calling for the ouster of the longtime president, who had been recuperating from injuries he received in a June attack on his palace. He has vowed to finish his term.
Government forces shot in the air to celebrate his return as tanks patrolled the streets of Sanaa. By noon, a massive crowd had gathered in central Sanaa to rally in support of the president.
Fierce clashes erupted in Sanaa between Republican Guards and soldiers under the command of a defector, Gen. Ali Mohsen. The fighting occurred mainly in the Hasaba and Dayeri districts.
Earlier in the day there were isolated clashes between anti-government fighters and Republican Guard forces in at least four areas in Sanaa, Yemen's capital.
Medics in Sanaa's Change Square report 12 deaths in Sanaa and one in the city of Taiz since midnight. There were 18 other injuries, they said.
The deaths are not from military fire but as a result of snipers and explosions hitting protesters, they said. No casualties have been reported from the military clashes.
Saleh has called for a cease-fire for all factions to ease the way for political dialogue and reach an agreement.
"Saleh stressed that the solution is not in the barrels of guns and cannons, but in dialogue, understanding and stopping the bloodshed and protecting life and preserving the security, stability and resources of the country," the state-run news agency reported.
State Department spokeswoman Victoria Nuland said the U.S. position on Yemen hasn't changed: America wants a peaceful transition of power.
"We want to see Yemen move forward on the basis of the GCC (Gulf Cooperation Council) proposal, and whether President Saleh is in or out of the country, he can make this happen by signing this accord, stepping down from power, and allowing his country to move on," she said.
Mohammed Qahtan, the spokesman for Yemen's opposition Joint Meeting Parties, said the president's return "does not affect us" and "the revolution will continue peacefully until all its goals are accomplished."
Yemeni activist and blogger Atiaf Alwazir said she is concerned.
"It's terrible news," Alwazir said of Saleh's return. "It's shocking because our concern was that he would try to place his son in charge. We didn't think he would return. We're very tense and we don't know what to expect. It's the polarization in the country that is so worrying right now. Whether he steps down or not, we don't know what to expect because the country is so polarized right now."
Protests Friday were a continued the demonstrations against Saleh that have stretched on for months.
Demonstrators began protesting Saleh's 33-year-old regime on February 11 inspired by the revolution in Egypt.
A month later, Saleh offered to draft a new constitution that would establish a parliamentary system, but protesters persisted in calling for his resignation, and numerous high-ranking political and military officials resigned or were dismissed.
Saleh balked after making overtures to accept an agreement with the Gulf Cooperation Council to step down.
Amnesty International said that since February, 200 people have been killed and more than 1,000 injured in the protests.
Government officials have repeatedly denied accusations of excessive use of force, and said the government is committed to establishing a peaceful transfer of power.
Yemeni officials have said forces cracked down on those committing acts of violence during protests."
CNN's Mohammed Jamjoom and journalist Hakim Almasmari contributed to this report.