Reviled by many as the spokesman for the Irish Republican Army in the 1980s during its campaign against British rule, Adams reinvented himself as a Northern Ireland peacemaker and then as a populist opposition parliamentarian in the Irish Republic.
His Sinn Fein party, which shares power in the Northern Ireland government, said he was arrested in the town of Antrim on Wednesday evening by police investigating the 1972 abduction and murder of Jean McConville, a widowed mother of 10 children.
Adams can be held for up to 48 hours, or until 8 p.m. (1900 GMT) on Friday, before a judge must rule on whether he can be detained any longer. Under British anti-terrorism laws, a suspect can be held for up to 28 days before being charged.
Adams, 65, who has always denied membership of the IRA, said he was "innocent of any part" in the killing, which he said was "wrong and a grievous injustice to her and her family".
"Well publicized, malicious allegations have been made against me. I reject these," he said in a statement.