* Upper house chairman Abdelkader Bensalah assumes post
* Protesters want radical changes after autocratic rule
* Thousands demonstrate against appointment (Adds quotes, details)
By Lamine Chikhi and Hamid Ould Ahmed
ALGIERS, April 9 (Reuters) - Algeria’s interim president, in a televised speech, promised on Tuesday to organise free elections within 90 days following weeks of protests that led to the resignation of leader Abdelaziz Bouteflika after 20 years in power.
Abdelkader Bensalah, the interim president, was rejected by demonstrators right after he was named by Parliament to take charge during a volatile transition period after decades of autocratic rule.
“I am committed to organising elections,” said Bensalah, who has been re-elected as leader of the upper house since the early 2000s. The army was aligned with the constitution as a pathway out of the crisis, he added in his 16-minute speech.
The critical question is how Algeria’s military, which has swayed Algerian politics from behind the scenes for decades, will react to Bensalah’s appointment and any opposition that arises.
Army Chief of Staff Lieutenant General Gaid Salah patiently managed Bouteflika’s exit after declaring him unfit to stay in power and expressed support for protesters, who have put up little resistance to the military.
Hours after Parliament made its choice, Salah said the military will do more to ensure peace for the Algerian people, state news agency APS reported.
Bensalah said he would consult with the political class and civil society. A long-time ally of Bouteflika, he is seen by protesters as part of an ageing and out-of-touch ruling caste that has dominated since independence from France in 1962.
Bensalah promised to “set a national and sovereign commission to secure fair elections” in an apparent bid to placate demonstrators demanding sweeping democratic reforms and economic opportunities. More than one in four people under the age of 30 - some 70 percent of Algeria’s population - are unemployed despite the country’s vast oil wealth.
The demonstrations, which erupted on Feb. 22, led to the disintegration of what has been described as the ruling elite’s “fortress” - veterans of the war of independence against France, ruling party figures, businessmen, the army and labour unions.
But Algerians want more radical change.
“You go means you go,” read banners at the protest in the capital on Tuesday, reiterating the desire of many Algerians to remove all remnants of a secretive political and military establishment that has dominated for decades.
On stepping down, Bouteflika promised that elections would be held after 90 days as part of a transition to usher in what he said would be a new era.
Reporting by Lamine Chikhi and Hamid Ould Ahmed Additional reporting by Aziz El Yaakoubi in Dubai Writing by Michael Georgy Editing by Raissa Kasolowsky, Giles Elgood, James Dalgleish and Leslie Adler