The chief of a leading Venezuelan opposition party on Sunday said a grenade exploded outside its Caracas headquarters. Democratic Action leader Henry Ramos said the blast shattered windows and damaged the facade of the party offices late Saturday. No one was hurt. Assailants have targeted other opponents of President Hugo Chavez in recent weeks with tear gas canisters thrown at buildings owned by government critics. Tensions are high ahead of a February 15 referendum that would end presidential term limits, letting Chavez seek re-election indefinitely.