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At least 11 leading members of the Republican Rally party - including the deputy leader, Henriette Diabate, and four members of parliament - were sentenced to two years in prison.
Security was tight around the commercial capital, Abidjan, for the verdict amid fears of opposition protests.
Political trial
Our West Africa correspondent Mark Doyle says the trial was essentially a political one, and government and the opposition remain far apart over the issue which led to the police crackdown on the opposition in the first place.
The opposition demonstration last month, which was accompanied by violent unrest, was called to protest about bias in the state-controlled media against the opposition's presidential candidate, Alassane Ouattara, who is currently outside the country.
The government argues that Mr Ouattara is ineligible to stand in next year's presidential elections because on the grounds that he is not Ivorian, and has accused him of falsifying his Ivorian identity.
Mr Ouattara, a former prime minister of Ivory Coast and a wealthy international banker, insists he is an Ivorian citizen and denies he falsified his papers.
The US state department called the trial an attempt to stifle democracy in Ivory Coast.
The ruling party in France - the former colonial power which remains influential in Ivory Coast - has also condemned the moves against the opposition.
Country profiles
Ivory Coast opposition leaders face trial
(04 Nov 99 | Africa)
Ivory Coast 'back to one-party rule'
(28 Oct 99 | Africa)
Opposition supporters arrested in Ivory Coast
(15 Sep 99 | Africa)
Ivory Coast bans student protest
(01 Jun 99 | Africa)
Africa News Online - Ivory Coast
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