By Tarek Amara

TUNIS (Reuters) - Tunisia's embattled Constituent Assembly met on Tuesday for the first time since one of its members was assassinated, ahead of planned mass protests calling for the body to be dissolved.

The Assembly is only weeks away from finishing a draft constitution and electoral law that would move the fledgling democracy closer to new elections.

The country's secular opposition is trying to oust the Islamist-led government and dissolve the transitional Assembly.

Protests have been held daily since the killing of leftist politician and Assembly member Mohamed Brahmi on July 25, nearly six months after another leftist figure was gunned down.

Tunisians are facing the worst political crisis since the toppling of autocratic ruler Zine el-Abidine Ben Ali in 2011, in a revolt that sparked uprisings across the Arab world.

The opposition has vowed to launch its biggest protest yet later on Tuesday to mark the sixth-month anniversary of the first politician to be assassinated since the revolt, Chokri Belaid.

More than 70 members of the Assembly withdrew two weeks ago in protest at the two killings and organised a sit-in outside the Assembly headquarters.

The Constituent Assembly met on Tuesday morning despite the absence of protesting lawmakers.

The atmosphere was particularly sombre - the empty chair of slain Brahmi was draped in a Tunisian flag and covered in flowers and a picture of him.

Prime Minister Ali Larayedh attended the meeting to urge the Assembly to finish its work, seen as key to imposing some kind of order not only on politics, but also on security.

Over the past two weeks, Tunisia has seen a spike in Islamist militant attacks that appear to be exploiting the political turmoil.

"I call on the Constituent Assembly to hurry and finish the constitution quickly and to complete all of its other tasks by October 23," Larayedh said.

In a an effort to appease the opposition, Larayedh promised last week to speed up the Assembly's work and hold fresh elections on December 17.

The opposition, which has been emboldened by the army-backed ouster of Egypt's elected Islamist president, says the government should be removed due to the worsening security and economic conditions.

It says the Assembly is well behind in its work - the body has already gone eight months beyond its promised deadline - and should therefore be scrapped.

But the ruling Ennahda party's chief Rachid Ghannouchi said removing Larayedh or dissolving the Assembly was a "red line", and challenged the opposition to a referendum to decide the matter.

In addition to opposition groups, the powerful Tunisian General Labour Union (UGTT) called on its followers to join Tuesday's anti-government protest.

The 600,000-strong union could play a game-changing role in the political crisis, given its ability to hold countrywide strikes that could paralyse much of the economy.

(Writing by Erika Solomon; editing by Mike Collett-White)