Army chief takes power after Burkina president bows to protests

* In statement, Compaore calls for election in 90 days

* Protesters frustrated after military chief takes power

* General vows "process of return to constitutional order" (Adds details)

By Mathieu Bonkoungou and Joe Penney

OUAGADOUGOU, Oct 31 (Reuters) - The head of Burkina Faso's armed forces took power on Friday after President Blaise Compaore resigned amid mass protests against an attempt to extend his 27-year rule.

Compaore, a taciturn former soldier in office since a 1987 military coup, had sought to defy popular pressure for him to step down after a day of violent unrest on Thursday in which demonstrators stormed parliament and state television.

Impoverished Burkina Faso under Compaore had emerged as a key mediator in the turbulent Sahel and his departure robs the region of an elder statesman -- though one whose rights record was often criticised.

The landlocked nation is also a key ally in Western operations against al Qaeda-linked groups in West Africa and the crisis was being closely watched by the United States and France, which has a special forces base there.

Demonstrations erupted on Thursday when parliament had been due to vote on plans to change the constitution to allow Compaore to seek reelection next year. At least three people were shot dead and dozens wounded.

With hundreds of thousands packing the Place de la Nation in the capital Ouagadougou for a second day on Friday, and with no sign of international support -- particularly from former colonial power France -- Compaore bowed to public pressure.

"I declare a vacancy of power with a view to allowing a transition that should end with free and transparent elections in a maximum period of 90 days," Compaore said in a written statement read by presenters on local radio and television.

CONVOY HEADS TOWARDS BORDER

A heavily-armed convoy believed to be carrying the former president was seen travelling towards the southern town of Po near the border with Ghana, which is home to a large military base, diplomatic sources and local media said.

The departure of the 63-year-old -- who until recently was seen as one of West Africa's most invulnerable Big Men -- will send ripples across a region where several long-standing rulers are nearing the end of their terms amid rumbling discontent.

Crowds danced, cheered and blew whistles in Ouagadougou's dusty streets after Compaore's statement was broadcast.

"This is a sub-Saharan Spring and it must continue against all the presidents who are trying to hang on to power in Africa," said law student Lucien Trinnou, referring to the Arab Spring that toppled several long-term leaders.

Jubilation turned to frustation, however, as it became plain that military chief General Honore Traore -- a former military aide de campe of Compaore -- had taken over the reins of power.

Under Burkina Faso's constitution, the head of the National Assembly should take office if the president resigns but parliament was dissolved by Traore on Thursday under short-lived martial law, leaving a power vacuum into which he stepped.

It was the seventh time that a military officer had taken power since the country declared independence from France in 1960, when it was known as Upper Volta.

"Considering the urgency of saving the nation, I have decided to assume from today the responsibilities of head of state," said the bespectacled Traore, wearing military fatigues and a three-star general's cap, flanked by other officers.

"I make a solemn pledge to proceed without delay to consultations with all parties to start the process of returning to the constitutional order as soon as possible."

There was no immediate reaction from the leaders of the fragmented opposition to the announcement by Traore, regarded by many as a Compaore loyalist. He was named military chief in the wake of a failed 2011 uprising in which soldiers took part, with a mission to shake up the armed forces.

In the Place de la Nation, the epicentre of the demonstrations, many people called for a transition led by retired General Kouame Lougue, a popular former defence minister accused of trying to topple Compaore in 2004.

"The people have seized power and chosen its president. It is General Kouame Lougue. We don't trust Honore Traore who was a lackey to Blaise Compaore," said a demonstrator who identified himself as Souleyman. Others chanted "Lougue, Lougue!"

PRESIDENTIAL GUARD SPEAKS OUT

Adding to the uncertainty, Lieutenant Colonel Issaac Zida -- the presidential guard commander who had first announced Compaore's decision to resign -- read his own statement to the crowd at a military barracks surrounded by armed troops.

"A transitional body will be put in place in consultation with all stakeholders in the nation," he said, announcing the suspension of Burkina's 1991 constiution.

It was not immediately clear if this was a challenge to Traore's authority from within the presidential guard, the best-equipped and trained section of the armed forces and traditionally regarded as fiercely loyal to Compaore.

French President Francois Hollande, who had discreetly sought ways to usher Compaore into an international role when his term ended next year, welcomed the former president's resignation in a statement and called for quick elections.

A delegation from the African Union, the United Nations and regional West African bloc ECOWAS was due in Burkina Faso on Friday to hold talks with all sides. ECOWAS said on Thursday it would not accept any seizure of power by unconstitutional means.

The country was also of the last in Africa to retain diplomatic relations with Taiwan, and diplomats will closely watch for any change of foreign policy stance.

Compaore is often blamed for the death of former president Thomas Sankara -- his friend and former patron -- in his 1987 putsch. Sankara was a popular left-wing revolutionary, dubbed "Africa's Che Guevara", who changed the name of the country to Burkina Faso -- meaning 'Land of the Upright Men'.

The political turbulence drew a sanguine response from mining investors in Africa's fourth largest gold producer, including Endeavour Mining, IamGold (Xetra: 899657 - news) and Randgold Resources.

"We are hopeful that there will be some kind of peaceful transition," said Jim Wynn, vice president for finance and company secretary at Avocet, which operates a gold mine in the northeast. "It shouldn't affect our operations unless it gets significantly worse and we hope that it won't." (Additional reporting by John Irish in Paris, Emma Farge and Bate Felix in Dakar; Writing by Matthew Mpoke Bigg and Daniel Flynn; Editing by Emma Farge and Andrew Roche)