CAIRO — Egypt descended deeper into political turmoil on Thursday as the embattled president, Mohammed Morsi, blamed an outbreak of violence on a ‘‘fifth column’’ and vowed to proceed with a referendum on an Islamist-backed constitution that has prompted deadly street battles between his supporters and their secular opponents.

As the tanks and armored vehicles of an elite military unit ringed the presidential palace, Morsi gave a nationally televised address offering only a hint of compromise, while preserving his assertion of sweeping authority. His opponents quickly rejected, even mocked, his speech and vowed continued protests ahead of a planned Dec. 15 vote on the draft constitution.