Police from the Czech Republic's organised crime unit detained a senior aide to Prime Minister Petr Necas on Thursday, local media reported, after raids around the capital that rocked the foundations of an already-fragile government.

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 organised crime unit confirmed the unit was involved in an operation but refused to provide any details. Police called a news conference for 3.30 p.m. (1330 GMT).

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 detained in the raids. Nagyova is a long-standing associate of the prime minister.

Parliament interrupted its session to seek clarification about what was happening, and the president - a political opponent of Necas - called a meeting with the prime minister and police chief.

The 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.

The Czech Republic was a beacon of liberty in 1989 when former dissident Vaclav Havel led a Velvet Revolution against communist rule. But since then successive governments have been dogged by accusations and rumours of corruption -- though none has ever led to high-profile convictions.

Necas's cabinet has won praise for appointing prosecutors with a free hand to pursue corruption cases in the country, an EU member state of 10.5 million people, but his cabinet has suffered a series of scandals itself.

Peter Honzejk, a commentator at the daily Hospodarske Noviny, said the raids and the report that Nagyova has been detained was the most serious blow to Necas to date.

"I think Petr Necas cannot survive this," he said. "Everyone knows how close she (Nagyova) is to him," he said.

If the government falls, a snap election may be called. The government's term is not scheduled to end until May next year.

No word from Necas

Government headquarters would not transfer telephone calls to Nagyova's office. The prime minister himself made no public appearances on Thursday and his spokesman was not answering his telephone. The parliament speaker said Necas would attend a parliamentary session on Friday morning.

Czech media reported that the police raids, which started on Wednesday night and were still underway on Thursday, targeted several locations around and outside the capital.

Police were present at the defence 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.

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.