Troops loyal to Yemeni President Ali Abdullah Saleh and forces opposed to his rule were engaged in heavy fighting across much of the capital Sanaa on Monday, with rockets, mortars and heavy machine-guns being used.
The fighting began shortly after midnight and intensified around 3 a.m. local time, with the sound of explosions rocking many parts of the city.
There were no overall casualty figures immediately available, but at least three people were killed in the downtown encampment housing tens of thousands of protesters demanding Saleh's ouster, according to medical officials. Six people were also wounded when a shell hit their house in an area in the northern part of the capital.
The officials spoke on condition of anonymity because they were not authorized to share the information.
The protesters in the encampment rushed to nearby buildings to find shelter from the shelling.
The fighting died down when the call for the Muslim dawn prayer rang out from the city's mosques, but resumed a short time afterward.
Pro-regime forces have frequently fought with rival tribesmen and renegade troops of the 1st Armored Division in Sanaa, but Monday's fighting was the most intense in weeks, with the pro-regime forces shelling their rivals' positions from the hills outside the city.
Saleh, Yemen's leader of 33 years, has clung to power in the face of eight months of massive protests across Yemen calling for his ouster. In March, a powerful tribal chief and a longtime Saleh ally joined the opposition along with several key army commanders, lawmakers and Cabinet ministers.
He has balked at signing a U.S.-backed deal proposed by his wealthy Gulf Arab neighbors that provides for him to transfer power to his deputy and step down in exchange for immunity.
According to a Yemeni foreign ministry official, the six-nation Gulf Cooperation Council, which initiated the power transfer deal, has turned down a request by Saleh to make changes to the proposals to enable him to stay in power until 2013. The official spoke on condition of anonymity because of the sensitivity of the matter.