* Interior minister says referendum to take place
* Parliamentary election also to go ahead on Aug. 20
* Government warns opposition over disobedience call
By Abdoulaye Massalatchi
NIAMEY, June 28 (Reuters) - Niger will hold a referendum on whether President Mamadou Tandja can extend his rule on Aug. 4 despite a decision by the country's highest court that it would be illegal, the government said on Sunday.
Tandja's plan to hold onto power has brought protests by unions and drawn criticism from foreign donors and regional political bodies, which call them a step backwards for the uranium-producing West African country.
Tandja is due to step down when his second term ends later this year, but wants a referendum which could let him stay three more years. After the Constitutional Court again rejected the plan on Friday, he announced he would rule by decree.
Interior Minister Albade Abouba said on Sunday that both the referendum and an Aug. 20 parliamentary election would go ahead.
"The elections will very well take place, starting with the referendum," he said on state media. "The independent electoral commission cannot reject the referendum. It's job is solely to organise and supervise the elections."
The electoral commission has not refused to organise the referendum, but pointedly made no mention of it when it set the date for the parliamentary election.
Abouba sent a warning to Niger's opposition, which described Tandja's decision to assume sweeping powers as a coup and urged security forces to disobey him.
The landlocked former French colony of 15 million people, which stretches to the heart of the Sahara desert, hopes to become the world's second biggest producer of uranium. It is one of the poorest countries in the world.
The president says he needs the time to introduce a fully presidential system of government that will give the president more power and end current blockages in governance.
He also says people want him to complete large infrastructure projects, including a hydro-electric dam, an oil refinery and French energy giant Areva's CEPFi.PA Imouraren uranium mine. (Editing by Matthew Tostevin)