UPDATE 6-Protests force out Burkina president, soldiers vie for power
* In statement, Compaore calls for election in 90 days
* Protesters frustrated after military chief claims power
* General vows "process of return to constitutional order"
* Lieutenant Colonel appears to challenge military chief (Adds U.S. comment in paragraph 6)
By Mathieu Bonkoungou and Joe Penney
OUAGADOUGOU, Oct 31 (Reuters) - Burkina Faso President Blaise Compaore resigned on Friday amid mass street protests at his attempts to extend his 27-year rule, plunging the West African nation into uncertainty as junior officers vied with top military brass for power.
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 burned parliament and ransacked state television.
Impoverished Burkina Faso had emerged under Compaore as a key mediator in the turbulent Sahel and his departure robs the region of a wily elder statesman -- though one often criticised for his rights record and meddling in his neighbours' affairs.
The landlocked nation has become a key ally in Western operations against al Qaeda-linked groups in West Africa and the unfolding political crisis was being closely watched by the United States and France, which has a special forces base there. Continued...