HONG KONG — The Hong Kong police on Thursday arrested five executives at an embattled pro-democracy newspaper on suspicion of colluding with foreign powers to violate the Chinese territory’s sweeping national security law.

The police said they had arrested five people of a media company on suspicion of “collusion with a foreign country or with external elements to endanger national security.” They did not name the company or specify who the foreign powers might be.

The five executives work for Apple Daily, a pro-democracy newspaper or its parent company, Next Digital, the newspaper reported. Apple Daily said that Ryan Law, its editor in chief, was among those arrested.

Next Digital’s founder, Jimmy Lai, is serving a prison sentence for illegal assembly and is facing charges under the national security law, which was imposed last year on Hong Kong by the mainland Chinese government.

Footage broadcast by Apple Daily reporters showed dozens of police officers entering the headquarters of Next Digital. Some carted away suitcases. Others set up makeshift posts at the entrance, requiring employees to register personal details. The officers restricted employee access to certain floors of the building and warned them to turn off their cameras.

The police said that a search warrant had authorized “the power of searching and seizure of journalistic materials,” but they did not describe what had been seized.

Mr. Lai and his newspaper have long been a thorn in the side of the Chinese and Hong Kong governments, and Beijing has targeted him as one of its leading enemies in the territory.

In the year since the national security law went into effect, the police have arrested 112 people in national security investigations, and 57 have been charged under it.