National Union of Journalists chief Jeremy Dear said Saturday the 48-hour walkout, which began Friday, had forced some news programs off the air.
The dispute is over planned pension changes that have been rejected by the union, which has 4,100 members at the BBC.
BBC director-general Mark Thompson insists the new pension plans are fair and have already been modified after talks with staff. He said only one in six employees had joined the strike.
The BBC’s flagship “Today” radio program was canceled Friday but broadcast as normal on Saturday.