Niger opposition accuses president of coup

* Demand for President Tandja's resignation

* Security forces urged to disobey president

By Abdoulaye Massalatchi

NIAMEY, June 27 (Reuters) - Niger President Mamadou Tandja's opponents accused him of carrying out a coup on Saturday and called on security forces to defy his orders after he announced he would rule the West African country by decree.

The political crisis in the uranium exporting state deepened on Friday when Niger's highest court rejected Tandja's latest attempt to push through a referendum on extending his rule. He then said he was assuming sweeping powers.

"President Tandja has proclaimed himself a dictator," said the opposition coalition, the Front for the Defence of Democracy in a statement. It called on Niger's people to mobilise against "the coup d'etat carried out by the president of the republic."

The opposition urged the military and other security forces to disobey Tandja and demanded that he step down. There was no immediate response from government or the forces to the opposition's call.

Tandja, 71, is due to step down when his second term in office ends later this year but wants a referendum which could hand him another three years in office.

Tandja's plans have sparked protests by unions and drawn criticism from foreign donors and regional political bodies, which said they were a step backwards and threatened sanctions against Niger.

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.

Tandja said he would rule by decree from Friday to safeguard the interests of Niger's people.

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) 1.2 billion euro ($1.70 billion) Imouraren uranium mine. (Editing by Matthew Tostevin)