BEIRUT—Syrian rebels disrupted highways in Damascus and battled government forces in several neighborhoods where tanks and armored vehicles were deployed, activists and residents said, in the second day of what opposition fighters called a planned push on President Bashar al-Assad's seat of power.

The West and Russia, meanwhile, appeared headed for a showdown over a resolution at the United Nations Security Council. The U.S. and its Western allies are seeking a formulation that would threaten U.N. sanctions against the Syrian...