Poland's former interior minister, who had accused the government of illegal phone-tapping, has been arrested.
Mr Kaczmarek had been regarded as an ally of the prime minister
Lawyers for Janusz Kaczmarek say he is alleged to have obstructed justice.
Mr Kaczmarek was fired earlier this month on suspicion of leaking confidential information to hinder a corruption inquiry.
The former national police chief and the head of a state-run insurance firm were also arrested. Opposition politicians condemned the arrests.
Last week, Mr Kaczmarek told a parliamentary commission that Prime Minister Jaroslaw Kaczynski and other top officials had used the secret services to spy on political opponents and journalists - allegations Mr Kaczynski denies.
The former police chief, Konrad Kornatowski, was due to testify before a secret services committee on Friday.
Opposition politicians say Mr Kaczynski is trying to silence critics ahead of a planned snap election.
"We don't think it is a coincidence that Kornatowski's arrest took place exactly a day before his planned hearing in front of the secret services commission," said the commission's head, Pawel Gras, quoted by the Associated Press news agency.
Before his dismissal, Mr Kaczmarek had been regarded as a close political ally of Prime Minister Kaczynski, the BBC's Adam Easton reports from Warsaw.
Mr Kaczynski said on Thursday the arrests were not politically motivated.
"These [arrests] occurred in the normal run of the justice system, and placing them in any [other] context is groundless," he said.
Mr Kaczynski's conservative coalition collapsed last month, and parliament will vote next week on his plans to call an early general election.