SANAA, Yemen — Pro-regime forces, including snipers picking off protesters from rooftops, killed at least 23 people Monday in a second day of clashes shaking Yemen’s capital, medical and security officials said.

The two days of fighting, which have killed nearly 50 people altogether, marked the most serious outbreak of violence in months, as frustration in the streets again builds over the president’s refusal to step down after 33 years in power.

The officials said thousands of protesters armed with sticks overran a camp belonging to the Presidential Guards in Sanaa and that others were headed toward the headquarters of the elite force led by President Ali Abdullah Saleh’s son Ahmed in the south of the city.

The officials said 20 of the 23 killed on Monday were on Sanaa’s central Hayel street. They included a child and at least three soldiers who defected to join the protesters. Mortar shells thought to have been fired by pro-regime forces killed another two people in the capital, said the officials, who spoke on condition of anonymity because they were not authorized to share the information.

Tens of thousands of protesters demonstrated in Sanaa Sunday to press demands for Saleh to step down. Pro-regime snipers killed at least 26 of the protesters.

Beside those killed, scores of protesters suffering gunshot wounds were taken to hospitals in Sanaa, according to Mohammed al-Maqtari, a doctor at a field hospital set up by the protesters. The wounded included soldiers from the 1st Armored Division, which, along with its commander, joined the protesters more than six months ago.

Witnesses said the soldiers were involved in skirmishes with the Presidential Guards.

In the southern city of Taiz, at least one protester was killed and 15 others were wounded Monday in clashes between anti-regime demonstrators and security forces, according to witnesses. In the southern port city of Aden, three protesters were wounded in clashes with government forces, witnesses there said.

Yemen’s protest movement has stepped up demonstrations in the past week, angered after Saleh deputized Vice President Abed Rabbo Mansour Hadi last week to negotiate further on a Gulf-mediated, U.S.-backed deal under which he would step down in return for immunity from prosecution. Saleh has already backed away three times from signing the deal.

Many believe the move is the latest of many delaying tactics. Saleh has resisted calls to resign.

The United States once saw Saleh as a key ally in the battle against the dangerous Yemen-based al-Qaida branch, which has taken over parts of southern Yemen under cover of the political turmoil in the country. The U.S. withdrew its support of Saleh as the protests gained strength.