Niger to hold referendum despite court ruling

* 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.