Hong Kong is a semiautonomous Chinese territory with its own legislature, the Legislative Council. Elections for the next legislative session will be held in September, and the arrest comes about a month before the start of nominations.

The Independent Commission Against Corruption said in a news release that it arrested Mr. Leung and charged him with misconduct in public office. The agency said Mr. Leung had failed to declare his acceptance of the donation, which was received via Mr. Lai’s associate Mark Simon. The agency added that the case had arisen from a corruption complaint and that the inquiries that followed revealed Mr. Leung’s suspected offense. The commission declined to comment further on Thursday.

Mr. Leung, 60, said in a post on Facebook that commission officials came to his home about 6:30 a.m. on Thursday. He was released on bail and is scheduled to appear in district court on Friday for a procedural hearing.

Regarding the money, Mr. Leung said by telephone on Thursday afternoon, “I gave to the party, to a publishing house, I can’t recollect the memory, but I need to talk to a lawyer and defend myself now.” He said he could not discuss the matter further because he had not yet talked to his lawyer.

He pointed out that the charges came just ahead of nominations for election to the Legislative Council. “It’s almost like a smear, you know,” he said.