Secular opposition parties are demanding the dissolution of the parliament and the Islamist-led government.
“The people want to topple the regime!” people in the crowd shouted. “Ghannouchi, assassin, criminal,” others chanted, referring to Rachid Ghannouchi, leader of the ruling moderate Islamist Ennahda party that Brahmi’s family says was behind the killing.
Ghannouchi has denounced Thursday’s assassination as an attack on democracy.
In a counterdemonstration outside parliament, hundreds of Islamists chanted slogans condemning what they called an attempted coup against democracy.
The Interior Ministry issued a statement calling for calm.
The death of Brahmi, a secular opposition figure gunned down outside his Tunis home on Thursday, came five months after another secular leader, Chokri Belaid, was killed in a similar attack that stoked violent protests.
Brahmi was buried near Belaid’s tomb at the al-Jalez cemetery in central Tunis, and mourners carried portraits of both slain politicians.
Brahmi’s death further deepened divisions between Islamists and their secular opponents that emerged after President Zine el-Abidine Ben Ali was toppled in 2011 in the first of the revolutions that also felled leaders in Egypt, Libya and Yemen.
Witnesses said one man was killed early on Saturday in an anti-government protest in the southern city of Gafsa. Violence also broke out in several other cities.
A bomb in a police car exploded in Tunis but caused no casualties, as authorities eager to maintain stability cast a nervous eye at events in Egypt, where violence has spiraled since the Islamist president was ousted by the military.