Iran’s supreme leader said Wednesday that questioning the results of Iran’s June presidential election is a crime, his strongest warning yet to opposition leaders who continue to insist the vote was rigged.

Ayatollah Ali Khamenei, who has the final say on all state matters in Iran, did not go so far as to order the arrest of those leaders, as called for by hard-liners, but his words signaled the government could take stronger action if the criticism continues.