By Saliou Samb

CONAKRY (Reuters) - Guinean media called a 24-hour strike for Thursday in protest at the government's closure of an opposition-owned radio station for one month in the run-up to an election after a listener had called on air for an uprising.

The long-delayed vote on June 30 is supposed to seal a transition to democracy after a 2008 military coup in the mineral-rich West African nation. But the opposition fears it will be rigged and has staged protests to try to block it.

The government ministers and senior opposition members met with a United Nations mediator on Monday, a first such meeting since the crisis began two years ago.

Political tension has alarmed many investors in Guinea, the world's largest exporter of bauxite, threatening to deter much-needed capital as it seeks to diversify its economy.

The state communications regulator suspended Planete FM radio on Thursday after a caller urged a revolt against President Alpha Conde. Planete FM, owned by opposition spokesman Aboubacar Sylla, cut the caller off and criticised his appeal.

"The National Communications Council's decision respects neither the law on freedom of the press nor the constitution," said Boubacar Yacine Diallo, head of the URTELGUI media association. "We demand that the CNC revoke this sanction."

Guinean law allows the closure of private media for a maximum of 72 hours for any act that might threaten state security.

International media watchdog Reporters Without Borders said in a statement the CNC was acting like an institution "outside the law" as its ruling was based on a 1991 statute which had been superseded by subsequent legislation.

More than 50 people have been killed in three months of political violence and ethnic clashes between pro-opposition Peuls and the Malinke group which supports the government.

Guinean media have reported an increasing number of violent attacks on journalists by both government and opposition supporters.

"Journalists have been attacked and threatened, radio stations have been attacked, all during political protests," Diallo said.

Conde has ordered an investigation into a round of protests in late May in which 12 people died in clashes between police and demonstrators.

During the meeting between the opposition and the government, U.N. mediator Said Djinnit said he was hoping both camps will accept to restart a dialogue that would create conditions for the election to hold.

"I invited the parties to come with an open mind to discuss all the issues on which there are differences, particularly of course on the electoral process," Djinnit said afterwards.

Opposition spokesman Sylla said they have decided to meet the government for a dialogue without preconditions. The parties will continue the closed-door meeting on Tuesday.

(Reporting by Saliou Samb; Writing by Bate Felix; Editing by Daniel Flynn and Angus MacSwan)