The lawyer, Roman Giertych, who has been involved in a series of high-profile cases against members of the governing Law and Justice party, was placed in handcuffs by a special anticorruption unit outside a Warsaw court. He has also represented prominent opposition figures, including Donald Tusk, the former president of the European Council.

Later, during a search of his home, Mr. Giertych fell unconscious on his bathroom floor and was rushed to the hospital. Further details about the incident were not immediately available from the authorities, but his daughter, Maria Giertych, said he had scuffled with an officer.

Mr. Giertych’s detention comes on the heels of one of the closest presidential elections in Poland since the nation ended communist rule in 1989, with the ruling party winning a narrow victory. And in the bitterly divided country, the opposition was quick to argue that the detention was evidence that the ruling party was using the country’s legal system to achieve political aims.

The ruling party “made it fashionable to make a show out of putting handcuffs on innocent people,” Radek Sikorski, an opposition lawmaker, told TVN24, a Polish televsion station.

Mr. Sikorski said the detention was designed to distract the public as anger builds over the government’s handling of the pandemic and to intimidate both lawyers and judges.

Late Thursday, Stanislaw Zaryn, a spokesman for the authorities, said Mr. Giertych was in “good condition” and undergoing medical examinations. But Jacek Dubois, a lawyer who has worked with Mr. Giertych, contradicted that assessment and said he had been told by the family that Mr. Giertych’s condition was “very serious.”

A government spokesman, Piotr Müller, said he was unfamiliar with the details of the case. But, he added, “I understand the anti-corruption services had grounds for detention.”

Mr. Giertych was expected to be charged with “appropriating company funds and inflicting upon it financial losses of great proportions, as well as of money laundering,” said Anna Marszalek, a spokeswoman for the prosecutor’s office