* 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'sarmed forces took power on Friday after President BlaiseCompaore resigned amid mass protests against an attempt toextend his 27-year rule.
Compaore, a taciturn former soldier in office since a 1987military coup, had sought to defy popular pressure for him tostep down after a day of violent unrest on Thursday in whichdemonstrators stormed parliament and state television.
Impoverished Burkina Faso under Compaore had emerged as akey mediator in the turbulent Sahel and his departure robs theregion of an elder statesman -- though one whose rights recordwas often criticised.
The landlocked nation is also a key ally in Westernoperations against al Qaeda-linked groups in West Africa and thecrisis was being closely watched by the United States andFrance, which has a special forces base there.
Demonstrations erupted on Thursday when parliament had beendue to vote on plans to change the constitution to allowCompaore to seek reelection next year. At least three peoplewere shot dead and dozens wounded.
With hundreds of thousands packing the Place de la Nation inthe capital Ouagadougou for a second day on Friday, and with nosign of international support -- particularly from formercolonial power France -- Compaore bowed to public pressure.
"I declare a vacancy of power with a view to allowing atransition that should end with free and transparent electionsin a maximum period of 90 days," Compaore said in a writtenstatement read by presenters on local radio and television.
CONVOY HEADS TOWARDS BORDER
A heavily-armed convoy believed to be carrying the formerpresident was seen travelling towards the southern town of Ponear the border with Ghana, which is home to a large militarybase, diplomatic sources and local media said.
The departure of the 63-year-old -- who until recently wasseen as one of West Africa's most invulnerable Big Men -- willsend ripples across a region where several long-standing rulersare nearing the end of their terms amid rumbling discontent.
Crowds danced, cheered and blew whistles in Ouagadougou'sdusty streets after Compaore's statement was broadcast.
"This is a sub-Saharan Spring and it must continue againstall the presidents who are trying to hang on to power inAfrica," said law student Lucien Trinnou, referring to the ArabSpring that toppled several long-term leaders.
Jubilation turned to frustation, however, as it became plainthat military chief General Honore Traore -- a former militaryaide de campe of Compaore -- had taken over the reins of power.
Under Burkina Faso's constitution, the head of the NationalAssembly should take office if the president resigns butparliament was dissolved by Traore on Thursday under short-livedmartial law, leaving a power vacuum into which he stepped.
It was the seventh time that a military officer had takenpower since the country declared independence from France in1960, when it was known as Upper Volta.
"Considering the urgency of saving the nation, I havedecided to assume from today the responsibilities of head ofstate," said the bespectacled Traore, wearing military fatiguesand a three-star general's cap, flanked by other officers.
"I make a solemn pledge to proceed without delay toconsultations with all parties to start the process of returningto the constitutional order as soon as possible."
There was no immediate reaction from the leaders of thefragmented opposition to the announcement by Traore, regarded bymany as a Compaore loyalist. He was named military chief in thewake of a failed 2011 uprising in which soldiers took part, witha mission to shake up the armed forces.
In the Place de la Nation, the epicentre of thedemonstrations, many people called for a transition led byretired General Kouame Lougue, a popular former defence ministeraccused of trying to topple Compaore in 2004.
"The people have seized power and chosen its president. Itis General Kouame Lougue. We don't trust Honore Traore who was alackey to Blaise Compaore," said a demonstrator who identifiedhimself 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 announcedCompaore's decision to resign -- read his own statement to thecrowd at a military barracks surrounded by armed troops.
"A transitional body will be put in place in consultationwith all stakeholders in the nation," he said, announcing thesuspension of Burkina's 1991 constiution.
It was not immediately clear if this was a challenge toTraore's authority from within the presidential guard, thebest-equipped and trained section of the armed forces andtraditionally regarded as fiercely loyal to Compaore.
French President Francois Hollande, who had discreetlysought ways to usher Compaore into an international role whenhis term ended next year, welcomed the former president'sresignation in a statement and called for quick elections.
A delegation from the African Union, the United Nations andregional West African bloc ECOWAS was due in Burkina Faso onFriday to hold talks with all sides. ECOWAS said on Thursday itwould not accept any seizure of power by unconstitutional means.
The country was also of the last in Africa to retaindiplomatic relations with Taiwan, and diplomats will closelywatch for any change of foreign policy stance.
Compaore is often blamed for the death of former presidentThomas Sankara -- his friend and former patron -- in his 1987putsch. Sankara was a popular left-wing revolutionary, dubbed"Africa's Che Guevara", who changed the name of the country toBurkina Faso -- meaning 'Land of the Upright Men'.
The political turbulence drew a sanguine response frommining investors in Africa's fourth largest gold producer,including Endeavour Mining, IamGold andRandgold Resources.
"We are hopeful that there will be some kind of peacefultransition," said Jim Wynn, vice president for finance andcompany secretary at Avocet, which operates a gold minein the northeast. "It shouldn't affect our operations unless itgets significantly worse and we hope that it won't." (Additional reporting by John Irish in Paris, Emma Farge andBate Felix in Dakar; Writing by Matthew Mpoke Bigg and DanielFlynn; Editing by Emma Farge and Andrew Roche)
