Tunisian president Moncef Marzouki's secular party is quitting the country's coalition government in anger at the dominant Islamist party's handling of the recent political crisis.
The move by the Congress for the Republic party threatens to deepen the crisis, prompted by the killing of an opposition leader last week.
Mr Marzouki is a long-time human rights activist whose ascension to the presidency was seen as a sign of Tunisia's democratic progress after it overthrew its authoritarian president, Zine El Abidine Ben Ali, in 2011.
The state news agency TAP reported that the Congress for the Republic party said it is quitting the coalition government, which is led by Islamist party Ennahda.
The centre-left Congress for the Republic party had wanted negotiations on a new government. Ennahda dominated Tunisia's first free elections, although Mr Marzouki's party and the secular Ettakatol party also held some government seats.
Fringe violence by radical Islamists has mounted in Tunisia, and secular critics of Ennahda accuse the party of not doing enough to stop the extremists.
The fatal shooting on Wednesday of opposition leader Chokri Belaid - one of Ennahda's most outspoken critics - sparked nationwide protests and calls for a new government.
The prime minister Hamadi Jebali, an Ennahda member, wants to form a new government of non-political technocrats. But Ennahda party leadership rejects that idea, with a crucial party council is meeting later today to discuss what to do.
After three days of street violence, the capital Tunis was relatively quiet, under the watchful eye of riot police.
Leading academic professor Khaled Adouani has expressed hope that the government would find a "disciplined" solution to the crisis, noting that a pro-Ennahda rally in Tunis ended peacefully. "We must not in these delicate circumstances think about anything else other than legality," he said.