SANAA, Yemen — Gunmen in civilians clothes opened fire on anti-government protesters in Yemen’s capital, and tanks shelled residential neighborhoods in another major city, killing a 13-year-old boy and leaving at least a dozen people wounded Thursday, witnesses and a medical official said.

The attacks came as thousands of activists marched in the capital Sanaa and in the central city of Taiz, calling for President Ali Abdullah Saleh to face trial for his government’s crackdown on protesters.

For nearly 10 months, protesters have filled streets and public squares across Yemen, calling for the ouster of Saleh, who has been in power for three decades. While Saleh’s security forces have used deadly force to suppress the protests, international diplomacy has failed to resolve the crisis.

The U.N. Secretary General’s special envoy to Yemen, Jamal bin Omar, arrived in the country Thursday to seek progress on a U.S.-backed proposal to end the crisis. The plan was put forward by Yemen’s powerful Gulf Arab neighbors.

Under the plan, Saleh would step down and pass power to his vice president in exchange for immunity from prosecution. Saleh has agreed to the proposal three times, only to refuse to sign at the last minute.

Many Yemeni activists criticize the proposal for granting Saleh immunity and allowing him to stay on as head of the ruling party. They complain that it falls short of the democratic reforms and new government they want their uprising to achieve.

Thousands took to the streets in Sanaa and Taiz Thursday to call for Saleh to stand trial for his government’s deadly crackdown.

In Yemen’s second largest city, Taiz, tanks shelled residential neighborhoods, destroying several buildings, and security forces opened fire on a protest in the city’s center, killing a 13-year-old boy and wounding nine others, a medical official said.

The medical official spoke on condition of anonymity because he was not authorized to brief reporters.

Printed on Friday, November 11, 2011 as: Yemeni protesters meet gunfire