Czech police on Thursday arrested Prime Minister Petr Necas's top aides and several former allies in an unprecedented anti-corruption swoop threatening the country's shaky government.
The raids sparked calls for Necas to resign and prompted his political rival President Milos Zeman to summon him, the national police chief and the opposition leader for crisis talks.
Four hundred policemen from an anti-organised crime squad launched raids at the cabinet office and defence ministry, where they arrested "several people", said Pavel Hantak, spokesman for the organised crime police squad.
"It's a very massive swoop, a case we have been working on for many months," Hantak told reporters.
The DNES daily broadsheet reported Thursday evening that eight people had been charged in connection with five crimes following the raids, but did not provide further details.
Justice authorities were not immediately available for confirmation.
Earlier Thursday, Necas confirmed that Jana Nagyova, who headed his office, was arrested.
But he backed his top aide, saying his trust in her "has not decreased and I have no reason to think she has acted against the law".
"Right now I don't have information proving criminal acts committed by my co-workers. I expect the police and prosecutors to announce quickly the reasons... behind this extensive swoop," he said.
Ironically, Necas, a 48-year-old physicist, has made a high profile anti-corruption drive a centrepiece of his administration, which has been in power since July 2010. The next general election is due next May.
Bohuslav Sobotka, head of the left-wing opposition Social Democrats, insisted that Necas must step down and call snap elections.
"The Czech Republic can't have a prime minister whose closest allies are suspected of ties to mafia," he said.
"We think the only democratic and responsible solution to the current political crisis caused by the criminal problems of (Necas's) Civic Democrats is ... early elections, as soon as possible."
Sobotka's party has been tipped by pollsters to be a clear winner of potential snap elections.
Political analyst Jan Outly said that if "his assistant has been arrested, there's nothing to talk about and the prime minister must bear responsibility."
Necas dismissed the calls.
"I personally am convinced I haven't done anything dishonest, nor have my co-workers or Mrs Nagyova.
"Therefore I have no reason to consider a resignation and ... the government's fall," said Necas, who heads the wobbly centre-right coalition that has already survived seven confidence votes.
Necas's rival Zeman declined to comment on "media information which is often twisted" until Friday's crisis talks.
Interior Minister Jan Kubice told lawmakers that two prosecutors and the head of the organised crime squad had visited Necas "within criminal proceedings" at night, without giving further details.
DNES said police were also searching Prague city hall and the villas and offices of several influential businessmen.
Last month, Necas's government approved a bill giving state prosecutors more power when investigating graft in top politics. If approved by parliament, the law will take effect on January 1, 2015.
An EU member since 2004, the ex-communist Czech Republic has been plagued by corruption since it emerged as an independent state after its 1993 split with Slovakia.
Corruption watchdog Transparency International in 2012 ranked the Czech Republic 54th, behind countries such as Costa Rica and Rwanda.