Interior Ministry spokesman Mohammed Ali Aroui said gunmen shot dead Brahmi, who leads the leftist People Movement party, outside his house Thursday morning.
Brahmi, 58, was in his car outside his home when gunmen fired several shots at him, Aroui added.
Thousands of angry supporters of the slain opposition member reportedly poured onto the streets of Tunis and other cities to protest the assassination. The protesters gathered in front of the Ministry of Interior, demanding the closure of parliament.
The slain leader held a seat in the assembly tasked with writing the new constitution.
This is the first killing of an opposition politician since leftist Chokri Belaid was assassinated in February, triggering a political crisis in the North African country.
The government has blamed Belaid’s assassination on extremists.
Tunisia, the birthplace of pro-democracy protests across North Africa and the Middle East, is struggling with a democratic changeover after the overthrow of its dictator in 2011.
The moderate ruling party Ennahda was elected following the ouster of former dictator, Zine el-Abidine Ben Ali, in January 2011.
Tunisia has seen numerous clashes between the authorities and extremist groups over the past few months.