* Further blow to government after politician's killing
* Tunisia polarised between Islamists and their opponents
* Prime minister trying to form technocrat cabinet
By Tarek Amara and Alistair Lyon
TUNIS, Feb 10 (Reuters) - Tunisian President Moncef Marzouki's secular party withdrew on Sunday from an Islamist-led government already reeling from last week's assassination of secular opposition leader Chokri Belaid.
Belaid's killing on Wednesday - Tunisia's first such political assassination in decades - has thrown the government and the country into turmoil, widening rifts between the dominant Islamist Ennahda party and its secular-minded foes.
"We have been saying for a week that if the foreign and justice ministers were not changed, we would withdraw from the government," Samir Ben Amor, an official of Marzouki's Congress for the Republic Party (CPR), told Reuters.
The CPR has criticised the performance of the two ministers, one of whom, Foreign Minister Rafik Abdessalem, is the son-in-law of Ennahda party leader Rachid Ghannouchi.
Ben Amor said the CPR's withdrawal was unconnected to Prime Minister Hamadi Jebali's decision, announced after Belaid was killed, to form a non-partisan government of technocrats to run the country until elections can be held later in the year.
Senior politicians in Ennahda, as well as in its two non-Islamist coalition partners, had criticised Jebali's proposal, saying he had failed to consult them first.
Jebali said on Saturday he would unveil his new cabinet this week, but would resign if political parties did not support it.
A senior Ennahda official, who asked not to be named, said the National Constituent Assembly would have the final say, but added: "We see that it will be possible to form a government of technocrats that includes political parties."
Ben Amor said Marzouki's CPR would formally submit the resignation of its three ministers to Jebali on Monday.
Political analyst Youssef Ouslati said the party was "trying to jump out of a sinking ship", but that its decision had no great weight because Jebali was now the central player.
He said that if political uncertainty continued, "the street will be the crucial element".
Belaid's funeral drew tens of thousands of mourners in Tunis and other cities on Friday in what turned into mass political protests against Ennahda and the government it dominates.
About 6,000 Ennahda supporters took to the streets of the capital on Saturday in a peaceful show of strength.
The CPR's departure is the first major shake-up in the government set up in December 2011 after an election for a National Constituent Assembly to draft a new constitution.
The CPR came a distant second in the election, winning 29 of the assembly's 217 seats to Ennahda's 89, but Marzouki was elected interim president by the assembly in a show of unity and his party entered a coalition government led by Ennahda.
Marzouki had opposed former President Zine al-Abidine Ben Ali from exile until a popular uprising swept the long serving Tunisian leader from power in January 2011.
Since then Islamists and their opponents have tussled over the role of Islam in politics, society and the constitution, while economic grievances that helped drive the revolt against Ben Ali have gone largely unaddressed.
Belaid's killing, for which no one has claimed responsibility, shocked the nation of nearly 11 million.
The politician's widow Basma said on Saturday she was asking the government to protect her family with official protection.
Some members of Belaid's family have accused Ennahda of being behind the shooting, something the party denies.
Ghannouchi, Ennahda's leader, has threatened legal action against politicians or journalists pointing the finger at him, saying they were "exploiting the blood of the deceased for narrow political ends at the expense of the truth".