Mr Brahmi's wife has accused the ruling Ennhada party of being behind the killing
Tunisia's largest union has called for a general strike after the killing of opposition party leader Mohammed Brahmi in the capital, Tunis.
All Tunisair flights to and from Tunisia have been cancelled on Friday, the airline said in a statement.
Protesters gathered in cities across the country on the eve of the strike, calling for the government to resign.
Mr Brahmi, who led the Movement of the People party, is the second politician shot dead in Tunisia this year.
Prime Minister Ali Larayedh condemned his assassination, but said: "We are against all calls to dissolve the government to create a [power] vacuum."
In February, the murder of prominent secular politician Chokri Belaid sparked mass protests and forced then-Prime Minister Hamadi Jebali to resign.
Mr Brahmi, 58, was not as big a political figure as Mr Belaid, but he too was a leftist critical of the ruling Islamist Ennahda party, which came to power after the overthrow of long-term ruler Zine al-Abidine Ben Ali in January 2011.
Gunmen on a motorbike shot Mr Brahmi in his car outside his home in Tunis on Thursday morning. It is not known yet who was behind the attack.
As news of the murder spread, protesters gathered outside the hospital near the Ministry of Interior in central Tunis.
Reports emerged on Thursday evening that police there had used tear gas to disperse demonstrators who were throwing stones at security forces.
There were also reports of clashes with police in the southern city of Sfax, after protesters allegedly stormed a local government office in the southern city of Sfax.
And demonstrators were said to have attacked Ennahda's headquarters in Sidi Bouzid, Mr Brahmi's hometown and the birthplace of the Arab Spring revolutions, which have swept the Middle East.
Tunisia's prime minister earlier said Mr Brahmi's murder was aimed at taking advantage of the upheaval in Egypt, where President Mohammed Morsi was recently ousted after mass protests against him and his ruling Muslim Brotherhood.
The family of Mr Brahmi has accused the governing Islamist Ennahda party of being behind the killing.
The party did not respond to the claim, but released a statement expressing "sadness and shock" at the "cowardly and despicable crime".
There has been deep division in Tunisia between Islamists and secular opponents since Ennahda came to power
The party has faced growing popular unrest over a faltering economy and a rising extremist Islamist movement.
Correspondents say many Tunisians, particularly the young, complain that their quest for secular democracy has been hijacked by intolerant Islamists, including the Muslim Brotherhood which forms part of the current government.