Many people have been killed in clashes in the Yemeni capital, Sanaa, as protests continue against President Ali Abdullah Saleh.
Demonstrators camped out on Change Square - the focus of protests - have been caught in fighting between the army and dissident soldiers, a journalist in the city told the BBC.
Some unconfirmed reports say as many as 40 people have died across the city.
The violence follows Mr Saleh's return from three months in Saudi Arabia.
He received treatment there following an assassination attempt. Correspondents say his return raises the risk of all-out civil war.
Clashes in Sanaa have recently intensified as elite Republican Guards, led by President Saleh's son Ahmed, fight running battles with army units that have defected to the opposition and tribal fighters who support the protesters.
After Saturday's fighting, Mohammed al-Qabati, a doctor at a field hospital in the square, told AP news agency that 28 protesters and one soldier guarding them had been killed.
He said ambulances had been forced to leave bodies in the streets because of the fighting, and many injured were reaching the hospital by motorcycle.
Tents and buildings were set on fire, witnesses said.
A resident near the camp told Reuters news agency that government forces had used armoured vehicles and rifles.
"It was an intense fight. My house was shaking like crazy. There are no protesters there now - it's just armed people," the witness said.
Activists have been camped out in Change Square since January, demanding an end to Mr Saleh's decades-long rule.
Clashes were also reported in the north-west Sanaa, where a spokesman for the dissident soldiers told AP that 11 rebel troops had been killed.
Gulf monarchies renewed their so far unsuccessful call for President Saleh to sign a power transfer deal "immediately", on the sidelines of the UN General Assembly in New York on Friday, AFP reported.
In a statement carried by a Saudi state news agency, they "condemned... the use of weapons, especially heavy weapons, against unarmed protesters".
After his surprise return to Sanaa, President Saleh called for a ceasefire to stop violence which has already claimed about 100 lives this week, mainly of unarmed anti-government protesters.
He was quoted by the state news agency Saba as saying he was "carrying the dove of peace and an olive branch".
He flew back after having treatment in Saudi Arabia for injuries sustained in a rocket attack on the grounds of his presidential palace.
Mr Saleh was greeted by thousands of enthusiastic supporters, who staged a rally in Sanaa.
The US, whose officials were reported to have been taken by surprise by President Saleh's return, urged him to begin a transfer of power and arrange presidential elections.
"The Yemeni people have suffered enough and deserve a path towards a better future," White House spokesman Jay Carney said.