BBC journalists have mounted picket lines after launching a 24-hour strike in a row over jobs, threatening disruption to radio and TV news programmes.
Members of the National Union of Journalists (NUJ) walked out from midnight as part of a campaign against compulsory redundancies.
The union said jobs were set to be axed across the corporation, including BBC Scotland, Five Live, the Asian Network and the World Service. NUJ members started a work to rule last week and stepped up the action with a strike, which was supported by other unions.
The dispute began in Scotland but was escalated after the union said compulsory redundancies were being planned in other parts of the corporation. The BBC said it could not speculate on possible disruption to programmes.
Picket lines were mounted outside BBC offices across the UK, including London, Cardiff, Glasgow and Birmingham. Michelle Stanistreet, NUJ general secretary, said: "NUJ members across the BBC are taking action to defend jobs and quality journalism at the corporation.
"They are angry and frustrated at the poor decisions being taken at the top of the BBC - decisions that are leading to journalists being forced out of their jobs and quality journalism and programming compromised.
"Instead of making sure that the redeployment process works properly in all areas of the BBC, managers are prepared to waste public money on needless redundancies and sacrifice the livelihoods of experienced and talented journalists, at the same time as advertising other jobs externally. It's particularly disappointing that the BBC has failed to engage meaningfully in attempts to resolve this dispute - an abdication of responsibility for a public service broadcaster."
A BBC spokesperson said: "We understand how frustrating and difficult situations involving redundancies can be, but it is disappointing the NUJ have chosen to take this action.
"We are working hard to ensure that we succeed in getting staff redeployed wherever we can and will continue to work with the unions to ensure that their members receive the right redeployment support. Our priority is to deliver the scheduled services to viewers and listeners. It is too early to speculate on potential changes to the schedule or which services may be affected."
The union has asked the BBC for a moratorium on all job cuts for a six month period, to allow for talks and negotiation with the new director general. The NUJ said 7,000 jobs had been cut at the BBC since 2004, while a further 2,000 are being lost under cost-saving plans.