Pakistani police arrested at least 10 lawmakers belonging to the imprisoned former Prime Minister Imran Khan’s political party, in an hourslong raid on the Parliament building in Islamabad that began Monday night, officials said.

The police sweep was the first time in recent memory that Pakistani lawmakers have been arrested while at the Parliament and it intensified the political turmoil gripping the country over the past nearly three years. That crisis has pit Pakistan’s powerful military — long seen as an invisible hand guiding the country’s politics — against the still-strong political force of Mr. Khan and his die-hard supporters.

The overnight police raid began around 8:30 p.m. on Monday night, when dozens of police officers entered the building in the capital shortly after the end of a legislative session. As word spread of the raid, some lawmakers barricaded themselves inside their offices, while others were pulled from their cars by police officers as they tried to leave the premises, according to videos and witnesses.

The lights in the building went out late Monday night, going back on only after the police sweep ended around 2 a.m. on Tuesday.

The party members were arrested on charges related to antiterrorism laws, according to court documents and leaders of Mr. Khan’s party, Pakistan Tehreek-i-Insaf, or P.T.I. At least one lawmaker who had been arrested was released from custody late on Tuesday, court documents show.

The police did not immediately respond to a request for comment.

The raid seemed to signal the lengths to which the security establishment is willing to go to squash Mr. Khan’s party, analysts said, even into the halls of Parliament.