BBC Wales journalists are among members of the National Union of Journalists (NUJ) striking over pensions.
The union says proposed changes in the corporation's pension scheme mean journalists must pay more and work longer to get less when they retire.
The BBC's director general, Mark Thompson, said the 48-hour strike would not make the need for "radical pension reform" go away.
Members of the other BBC unions have voted to accept a revised offer.
Picket lines were formed Friday morning at most of the BBC's offices in Wales including Broadcasting House in Llandaff, Cardiff.
NUJ spokesman Rhodri Lewis said: "Ultimately whatever happens in the BBC today may well happen in the public sector in six or 12 months time.
"People have paid into their pensions, they deserve their pensions, they don't deserve to have the rules changed half way along."
Many news programmes were running with shorter durations as a result of the strike action.
Mr Thompson said the planned changes were difficult for staff.
"However, the people who lose out most in any strike action are the very people we are here to serve - our audiences," he added.
A further 48-hour strike is planned for 15 and 16 November.