Shiite rebels and Sunni militiamen clashed in Sana, the capital, for a second day on Friday in battles that have killed at least 120 people and forced thousands to flee their homes. In the past few months, the Shiite rebels, known as the Houthis, have surged from their stronghold in the north, taking a string of cities. They have tapped into discontent with the government, which this year lifted fuel subsidies and has been criticized as too slow to make political and economic changes. In recent weeks, Houthi protesters have set up sit-in encampments in Sana. Their main opponents have been Sunni Muslim militias and army units allied with the Muslim Brotherhood’s branch in Yemen, or tribal fighters sympathetic with the Brotherhood or Al Qaeda. On Friday, the Houthis attacked the headquarters of the state television network, witnesses said.