Updated

SANAA, Yemen -- Heavy fighting between government forces and defected military troops shook the Yemeni capital early Friday, killing two people in what could signal the start of a power struggle just days after autocratic President Ali Abdullah Saleh agreed to end his 33-year rule.

The clashes pitted Central Security forces commanded by Saleh's nephew, Col. Yehia Saleh, against troops from the First Armored Division, headed by Gen. Ali Mohsen al-Ahmar, who defected and joined the protesters in March. The troops fired machine guns and mortars, some of which landed on civilian homes and scarred the facades of buildings.

A security official said one soldier from each side was killed before the fighting stopped around dawn. He spoke on condition of anonymity because he was not authorized to brief the media.

The two units have clashed in the past, but Friday's fighting, near the home of Vice President Abed Rabbo Mansour Hadi, was the first showdown between military units since Saleh signed a U.S.-backed proposal Wednesday in the Saudi capital Riyadh. Under the agreement, Saleh transferred power to his vice president, who is to serve as acting president and until early presidential elections within 90 days.

If the deal holds, Saleh would be the fourth dictator pushed from power this year by the Arab Spring uprisings.

But Saleh's spokesman, Ahmed al-Soufi, added further confusion to what exactly the agreement seeking to end the country's nine-month political crisis means, saying Friday that Saleh has not given up his "constitutional duties" and remains in power.

On Friday, tens of thousands of Yemenis returned to the streets across the country to reject the power-transfer deal and call for Saleh's trial for crimes ranging from corruption to lethal crackdowns on protests. Yemenis first took to the streets in late January, inspired by popular uprisings against dictators in Tunisia and Egypt, and have faced harsh police action since. Hundreds have been killed.

The crisis has created a security vacuum across the country, leading to clashes between armed tribesmen and government troops in a number of areas. In the restive south, Yemen's active Al Qaeda branch has taken advantage of the vacuum to overrun entire towns.