By Ingrid Melander

BRUSSELS, June 10 (Reuters) - Tunisia must quickly take steps to open up its political system if it hopes to avoid spreading social unrest, opposition politician Nejib Chebbi said on Tuesday.

Youth protests over joblessness and rising living costs have broken out in the past two months in the southwest of the usually stable north African state, which is due to hold a presidential election next year.

"If the political system is not reformed, the country is exposed to risks," Chebbi told Reuters, referring to the riots. "It's a social explosion which risks expanding to other regions."

Chebbi, former general secretary of the main opposition Progressive Democratic Party, is the only top politician so far to announce his intention to run in the election. But a draft law would block him from standing in the polls.

President Zine al Abidine Ben Ali came to power in 1987 and won 94.4 percent of the vote in the 2004 election against three opponents. He has come under local and foreign pressure for more political openness in the country of 10 million.

Chebbi called for dialogue between authorities and civil society, more diversity of political forces in parliament and the opening up of Tunisian media.

"Reform of the political system for the election is vital for Tunisia," Chebbi said after a meeting with the President of the European Parliament Hans-Gert Poettering in Brussels.

"Without liberalisation of political life and without a reform of the legal framework, Tunisia will remain stuck," he said, adding that Tunisia would go "backwards" if it did not reform.

Ben Ali announced a constitutional change in March to let more candidates contest the 2009 election, but the change would block Chebbi because it limits candidacy to party leaders in office for two consecutive years.

"It's a law which locks the country, which excludes free competition," said the 60-year-old lawyer, who was prevented from running in the 2004 election because his party did not have a seat in parliament.

"Even if we cannot get the system to open up, what we will have done until the elections will strengthen the democracy camp ... to get the system to open shortly," he said.

President Ben Ali has yet to confirm he will stand for re-election.

(Editing by Matthew Tostevin)

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