TUNIS, Tunisia — The assassination of a second opposition politician in six months has piled the pressure on Tunisia's troubled Islamist-led coalition government, which came to power in the wake of the Arab Spring but is struggling to right the economy and rein in extremists.

With the country brought to a virtual standstill by a general strike and the revelation that the same gun was apparently used by an al-Qaida-linked Islamist extremist cell in the two assassinations, calls grew Friday for the 18-month-old transitional government to stand down.

On Friday six opposition parties holding 42 seats announced their withdrawal from the 217-seat national assembly and called for the government, elected in the aftermath of the overthrow of the country's long-time dictator, to be replaced by a national unity government tasked with finishing off the constitution and paving the way for fresh elections.

"We are withdrawing from the constituent assembly, which has lost its credibility, and are calling for the dissolution of a government that has failed, and tomorrow we will engage in an open sit-in in front of the assembly until it is dissolved," the parties announced in a statement issued during a late-night press conference.

Tunisia is considered the birthplace of the Arab Spring. Its revolution inspired pro-democracy uprisings across the Middle East and set an example for political cooperation when a coalition was formed between the Islamist Ennahda Party and two secular parties.

However, a troubled economy, rising Islamist extremists and the two political slayings have tarnished the government and fueled opposition calls for its dissolution.

"The assassination of Mohammed Brahmi is a failure of the government and a failure of its security policy," said political analyst Alaya Allani. "I think most of the political elite feel it is urgent after the assassination to dissolve the current government and replace it with a non-partisan, competent one."

The government's failure was driven home, said Allani, when the Interior Minister revealed in a press conference that not only was the same radical Islamist group behind the two assassinations, but that the same gun was used.

Lotfi Ben Jeddou said the gun used to shoot leftist politician Brahmi 14 times in front of his home was the same 9mm semi-automatic pistol that killed opposition politician Chokri Belaid back in February.

Brahmi's assailant was Boubakr Hakim, a 30-year-old weapons smuggler with Islamist sympathies who was also part of the al-Qaida-linked cell that assassinated Belaid, according to Ben Jeddou.

Critics of the government have wondered why after five months Belaid's killers had still not been brought to justice and worse that the assassinations were continuing.

The opposition has accused Ennahda of being overly tolerant of a rising radical Islamist trend in the country that has shown violent tendencies in its efforts to instill greater piety in what has long been known as one of the most secular countries in the Arab world.

The killing of Brahmi of the leftist Popular Current comes at a particularly sensitive time as Tunisia's drawn out transition is finally reaching its end with the debate on the constitution and amid rising hopes that fresh elections will be held by the end of the year.

To pass the constitution, which is still being hotly debated in the assembly, a two-thirds majority is required.

"It's high time to take into account what the population and different opposition groups are saying about how this government has failed to protect Tunisians," said Kamel Labidi, an analyst and free speech activist who expressed worry that the Islamists might not compromise. "I am afraid the hardliners in the Islamist movement are not inclined generally to work with anyone to lead the country toward democracy."

After the assassination of Belaid, anti-government protests erupted and Hamadi Jebali, the prime minister at the time suggested the formation of a government of technocrats. His own party rejected his offer and Jebali resigned.

In the wake of the latest assassination, Ennahda has remained firm once again in its insistence on remaining in power until the transition is completed and new elections held.

Ajmi Lourimi, a member of the Ennahda executive bureau, criticized the opposition for trying to use the crisis for its own ends.

"The demands of the opposition are not realistic or responsible ... they want to throw the country into a deeper crisis and take it into the unknown with disastrous consequences," he told The Associated Press. "The only solution is dialogue and consensus among all the parties and find compromises on our differences and finish the transition period with elections as soon as possible."

The opposition, as well as the main labor union that called the general strike, have shown little interest in dialogue. Among Friday's disruptions, flights and public transportation were cancelled.

Instead several groups and political parties announced the formation of a National Salvation Front to hold protests until the resignation of the government.

The number of people protesting in central Tunis and in front of the assembly was modest given the summer heat and the holy fasting month of Ramadan.

However, Brahmi's funeral on Saturday is expected to attract thousands.

A pro-government demonstration briefly marched down central Tunis' Bourguiba Avenue, the main site of the protests that brought down dictator Zine El Abidine Ben Ali in 2011. Echoing supporters of the Islamist government in Egypt that was overthrown by a coup earlier in July, the protesters hailed the "legitimacy" of the government in the aftermath of elections.

Unlike their counterparts in Egypt, however, Tunisia's Islamists have consistently shown a willingness to compromise with the country's powerful secular forces, including on key matters like keeping reference to Islamic law out of the constitution.

Political analyst Allani said in the wake of the latest assassination that Ennahda needs to make serious compromises in light of the fall of the Muslim Brotherhood government in Egypt.

"This time the facts have changed and the regional events oblige it to make concessions or lose its credibility and open the way for new tensions in society," he said.

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Schemm reported from Rabat, Morocco.