By Tarek Amara and Erika Solomon

TUNIS (Reuters) - Tunisia's secular opposition said on Sunday it might set up an alternative "salvation government" to challenge the Islamist-led ruling coalition and show its anger at the assassinations of two leftist politicians in six months.

Opposition leaders, who have also been emboldened by the Egyptian army's overthrow of an Islamist president this month, said they had no interest in reconciliation with the dominant Islamist Ennahda party.

"We will meet this evening to discuss creating a new salvation government and will study the possibility of nominating a new prime minister to replace this failed government," said Jilani Hammami, a leader of the Salvation laFront coalition and Tunisian Workers' Party.

"There is no longer any doubt that the time for it to go has now passed."

Tunisians are bracing for what many worry may be one of the most tumultuous periods in their transition to democracy since the toppling of autocratic President Zein El Abidine Ben Ali in 2011, a revolt that inspired uprisings across the Arab world.

Cabinet ministers are set to meet on Monday to discuss the growing political crisis, a government official said, and a message from the prime minister to the nation is also expected.

On Thursday, assailants shot dead leftist politician Mohamed Brahmi using the same gun, according to the government, as was used to kill secular opposition leader Chokri Belaid on February 6.

The opposition holds Ennahda responsible for Brahmi's killing, which the government has blamed on hardline Salafist militants. Some violent protests have erupted in the capital Tunis as well as provincial cities since Brahmi's assassination.

On Sunday, competing pro- and anti-government protesters waving red Tunisian flags assembled on opposite sides of a square outside parliament, as dozens of black-uniformed security men stood guard.

There was no immediate violence, but the interior ministry called on one of the protest groups to move to another area in order to prevent clashes.

Hundreds of opposition supporters sang the national anthem and shouted "Get out" on their side of the square. A smaller crowd of dozens of pro-Ennahda demonstrators chanted back: "Respecting government legitimacy is a duty."

At a news conference in the square, opposition figure Khamees Kaseela said 70 lawmakers had now withdrawn from Tunisia's transitional Constituent Assembly, which is trying to complete a new constitution. They are planning to stay in the square, although police dispersed them on Saturday night.

"This is an open sit-in and we won't leave until our demands are met. We will hold the government responsible for any attacks on us, or any blood spilled," Kaseela said, as families arrived to join the sit-in with food to break their Ramadan fast.

"TEARING AT THE SEAMS"

The opposition had already been mobilizing against the Islamist-dominated government, inspired by the mass protests in Egypt which prompted the army to remove President Mohamed Mursi.

Islamists and other critics say the opposition campaign is threatening stability during a fragile transition process.

"They tried the ballot box and when that failed, they decided to drag us into chaos. They're tearing at the seams of this country," said Khaled al-Fadlawy, an Ennahda supporter.

The parliament speaker said on Saturday the government was discussing a new power-sharing deal and urged those withdrawing from the 217-member Constituent Assembly to reconsider.

"It's not rational to throw in the towel just meters away from the finish line," Mustafa Ben Jaafar said in a speech.

Ben Jaafar, a member of a secular party in the Ennahda-led coalition, said the assembly was only weeks away from completing the new constitution. But the opposition position has hardened.

"The opposition completely rejects all efforts at reconciliation presented by the head of the Constituent Assembly in terms of expanding powers," Hammami told Reuters.

Protests outside the assembly in the Tunisian capital drew thousands of people on Saturday after Brahmi's funeral.

Police used teargas to disperse opposition demonstrations in Tunis as well as Sidi Bouzid, Brahmi's hometown and the cradle of the uprising against Ben Ali.

At the protest on Sunday, opposition supporters brushed off concerns about the risk of turmoil.

"The government had a year to fix the economy, to finish the constitution as it promised. It failed. Their time is up," said demonstrator Sabeha Waguibi, wrapped in a Tunisian flag.

"And now we're going to end this leadership, whether by protests or war."

(Editing by Alistair Lyon)