Tunisia's premier prepared to form a national unity Cabinet and the president canceled a visit to Egypt Thursday after a top opposition figure was assassinated, UPI reported. Prime Minister Hamadi Jebali called the killing of Chokri Belaid, one of Tunisia's best-known human rights defenders and a fierce critic of the ruling Islamist party, a "heinous crime against the Tunisian people, against the principles of the revolution and the values of tolerance and acceptance of the other." President Moncef Marzouki, who cut short his participation in a European Parliament session in Strasbourg, France, vowed to fight those who opposed Tunisia's political transition. On returning to Tunis, the capital, from Strasbourg, he said "a conspiracy against Tunisia lies behind the assassination of Chokri Belaid to threaten the country's security and stability and sow disorder." Marzouki also canceled a planned Thursday trip to Cairo to participate in the Organization of Islamic Cooperation summit so he could deal with the crisis, which brought thousands of protesters onto the streets outside Tunisia's Interior Ministry. Belaid was shot and killed by two unidentified gunmen Wednesday morning as he was getting into his car outside his home in Tunis, the Interior Ministry said. The killing -- the first officially confirmed political assassination since the Jan. 14, 2011, overthrow of former President Zine el-Abidine Ben Ali -- led to mass protests in Tunis and other cities, including Sidi Bouzid, birthplace of Tunisia's revolution and the ensuing Arab Spring uprisings that began with the self-immolation of a Sidi Bouzid street vendor Dec. 17, 2010, to protest the confiscation of his wares and harassment by a municipal official. Riot police in Tunis Wednesday fired tear gas into crowds, and plainclothes security officers beat protesters with nightsticks, witnesses said. At least one officer was killed, authorities said.