Necas said he would not consider resigning after the raids, but the investigation is a severe blow to his government, which does not have a stable majority in parliament and has already come close to collapsing several times.
Hundreds of officers from the organised crime unit, some armed and wearing balaclavas, conducted sweeps of the government headquarters and the Defence Ministry. They also searched safe deposit boxes in a bank, seized documents at the capital's city hall, and raided private homes.
One anti-graft campaigner said it was the biggest operation in the past 20 years to tackle corruption in the Czech Republic, a European Union member of 10.5 million people.
Among those detained was Jana Nagyova, the head of the prime minister's office who has been, in effect, his long-standing personal assistant.
Police refused to give details of their investigation or disclose the names of the people they had detained. Necas himself, in a statement to the media, revealed that Nagyova was among people from his office who had been detained.
The police operation triggered a political crisis, with parliament interrupting its session, the crown currency dropping slightly, and the president scheduling a meeting with Necas and the head of the national police for Friday.
Bohuslav Sobotka, head of the centre-left opposition Social Democrats said the country should hold an early election and his party would start consultations with political partners and the president to secure that aim. The opposition party needs allies, as it alone does not have enough votes to dismiss the cabinet.
"The Social Democrats demand an immediate resignation of the prime minister," Sobotka told reporters.
The Czech Republic became a beacon of liberty in 1989 when former dissident Vaclav Havel led the "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 a high-profile conviction.
In a statement to the media, Necas said: "I am personally convinced that I did not do anything dishonest and that my colleagues have not done anything dishonest either. Therefore I do not have any reason to consider a resignation and, thus, the fall of the government."
"GATEKEEPER"
Referring to his aide's detention, Necas said: "I can only comment that my confidence in her has not declined and I have no reason to believe that she committed anything illegal."
Interior Minister Jan Kubice said that Necas himself had been visited late on Wednesday by the head of the organised crime unit and two state attorneys. But he gave no further details and it was unclear what the visit was about.
Petr Honzejk, a commentator at the daily Hospodarske Noviny newspaper, said the raids were the most serious blow yet to the centre-right government led by Necas.
"I think Petr Necas cannot survive this," he said. "Everyone knows how close she (Nagyova) is to him."
A senior government official told Reuters that Nagyova was personally close to Necas for years, and acted as a gatekeeper for people who wanted to see the prime minister.
The government switchboard directed calls for Nagyova to the press department, which refused to comment.
It was Necas who tried to break with the pattern of corruption being swept under the carpet by appointing prosecutors with a free hand to pursue sleaze cases.
"This is clearly the biggest police operation concerning corruption ... in the past 20 years," said Radim Bures of anti-graft group Transparency International.
"For the past year and a half police and the state attorney's office have shown they are not scared."
Necas said that people from his office were detained apparently in relation to a case last year when three former deputies from his party quit parliament after abandoning a rebellion that could have toppled the government. Two of them were later given top positions at state firms.
Czech media said police also searched the premises of politically-connected businessmen.
There was no indication from police or prosecutors of what corrupt activities they suspect in the case, but Czech media and politicians have long talked about the country being plagued by sleazy practices.
These have included overpriced IT, legal, services and equipment procurement contracts at various levels of state administration, often won by companies with anonymous owners in off-shore tax havens.
