By Jan Lopatka and Robert Muller

PRAGUE (Reuters) - Police from the Czech Republic's organized crime unit on Thursday detained a senior aide to Prime Minister Petr Necas, local media reported, in raids on several government offices that were confirmed by police and ministry officials.

Necas himself was visited by the head of the police unit and two state attorneys, Interior Minister Jan Kubice told parliament, but the purpose of the visit was not clear.

Kubice said the visit was "in connection with a step in the criminal proceedings". He gave no further details.

A spokesman for the police's organized crime unit confirmed that the unit was involved in an operation but refused to provide any details.

But Czech Television and the online version of the largest Czech mainstream daily Mlada fronta Dnes said the head of Necas's office Jana Nagyova was arrested in the raids.

A detention of a close aide would be a serious blow to the prime minister, whose centre-right government does not have a stable majority in parliament and has nearly collapsed several times since Necas was elected to the job in 2010.

Government headquarters would not transfer telephone calls to her office.

Police were present at the defense ministry, a spokeswoman there said, and Prague city hall said detectives had taken away unspecified documents. There were also searches at the offices of two politically-linked businessmen, local media said.

Czech media reported that the police raids started on Wednesday night and were still underway on Thursday.

Parliament had interrupted its Thursday session to seek clarification about what was happening.

President Milos Zeman's office said he would meet Necas, the national police chief and the head of the opposition centre-left Social Democrats to discuss the situation on Friday.

The Social Democrats called a party leadership meeting for Thursday afternoon to discuss next steps. A small opposition party immediately called for an early election.

The prime minister himself made no public appearances on Thursday and his spokesman was not answering his telephone.

The Czech republic was a beacon of liberty in the late 1980s when former dissident Vaclav Havel led a Velvet Revolution against Communist rule. But since then successive governments have been dogged by accusations and rumors of corruption -- though none has ever led to high-profile trials.

(Additional reporting by Jana Mlcochova; Editing by Sonya Hepinstall)