The Ivory Coast Government says that 11 people have been arrested on suspicion of plotting to assassinate political, administrative and military figures.
Tension is returning to Abidjan
The announcement follows complaints by two main opposition parties that their members have been arrested or abducted.
They fear this signals a new round of political oppression, following a three month ban on public marches last week.
Police chiefs were unable to confirm the identity of those arrested, reports the BBC's Kate Davenport in Abidjan.
Despite a power-sharing government, Ivory Coast remains divided between the rebel-controlled mostly Muslim north and the Christian south, in the hands of President Laurent Gbagbo.
Alphonse Kossonou, a director of the former ruling PDCI party, was arrested late on Thursday, without explanation.
A close family member said armed men in uniform came to his home at 2200 GMT, saying they were acting on orders from the presidential guard.
Political parties say Gbagbo is not respecting the peace deal
"We weren't sure who they were. We were afraid he had been abducted," he said.
The PDCI, which has now confirmed that Mr Kossonou is being held by paramilitary gendarmes, believes this is a politically-motivated arrest.
"We are sure the arrest is linked to the demonstration we had been planning, calling on the government to fully apply the Marcoussis peace accord [which set up a power-sharing government]," said a high-ranking member of the party, who preferred to remain anonymous.
He said they feared a return to the shadowy death squads that operated for months in Abidjan, before the unity government was set up in March.
A spokesman for the RDR party of Alassane Ouattara has also said it fears a resumption of political oppression.
They have lodged a complaint with the minister of security, concerning the arrest of 12 of its party members and supporters, 10 on Friday and two on Saturday, in the Abobo district of Abidjan.
Eyewitnesses in Abobo described how the RDR sympathisers, mostly northern Muslim bus drivers, were sitting drinking tea on Friday evening, when a truck full of police arrived shooting in the air, and proceeded to round up as many of them as they could grab.
Just across the road, about 100 members of the Young Patriots group, linked to President Gbagbo's FPI party, were holding a meeting.
According to local residents, many of the Young Patriots were members of the GPP, the extreme militia group which was officially outlawed on Thursday night, after going on the rampage and attacking French-owned power and water companies last week.
The minister of security has been unavailable for comment, our correspondent says.