TRIPOLI Oct 15 (Reuters) - A Libyan faction opposed to the U.N.-backed government has taken over a parliamentary headquarters in Tripoli and called for a new government, in a challenge to Western plans to bring together competing armed groups.

Libya's internationally backed government, which has struggled to impose its authority on rival factions, condemned the takeover as a bid to scuttle its attempts to end the power struggle over the North African OPEC member.

Since the fall of Muammar Gaddafi in 2011, Libya has been caught up in factional fighting between various groups of former rebels who battled against Gaddafi and then steadily turned against each other in a struggle for control.

A presidential council of the Government of National Unity (GNA) arrived in Tripoli in March in the latest attempt to bring together rival factions who have operated competing governments in the capital and in the east of the country since 2014.

Late on Friday, leaders of a former Tripoli government said they had taken over the Rixos Hotel in the capital, where part of the U.N.-backed government is supposed to operate. The hotel was already controlled by an armed group loyal to them.

"The presidential council was given chances one after another to form the government, but it fails... and has become an illegal executive authority," former premier Khalifa Ghwail said in a statement.

Ghwail called for a new government to be formed with the former Tripoli government and its rival in the east, where hardliners also oppose the U.N.-backed administration.

The capital was calm on Saturday morning and there appeared to have been no fighting during the takeover of the Rixos, which was supposed to be the base for the State Council, a legislative body made up of Tripoli's former parliament as part of U.N.-backed unity government deal.

"The seizure of state council is an attempt to hinder the implementation of political agreement by a group which rejects this deal after it has proved its failure in managing the state," the presidential council said in a statement.

Tripoli is controlled by various armed brigades some loyal to the GNA and others who backed the former National Salvation government when its forces took over the capital in 2014 in fighting that destroyed the international airport.

Challenging the GNA's authority in the capital poses a risk to Western government plans for the unity government to stabilise Libya, and help fight Islamist militants and migrant smugglers.

Eastern factions led by former Gen. Khalifa Haftar are also opposed to the U.N.-backed Tripoli government. But they fought a conflict with rivals for control of Tripoli in 2014. Haftar's forces have taken over four key oil ports and now are cooperating for the moment with the GNA in allowing oil exports. (Reporting by Ahmed Elumami; writing by Patrick Markey Editing by Jeremy Gaunt)