A 24-hour strike by BBC workers brought disruption to TV and radio schedules today.
The BBC's flagship Today radio programme was the most high-profile victim of the industrial action.
Picket lines have been mounted outside Bush House in central London, TV Centre in White City, and Broadcasting House.
Thousands of journalists and technicians are taking part in the stoppage in protest at plans to axe BBC 4,000 jobs.
As well as Radio 4's Today programme, BBC1's Breakfast, BBC News 24 and BBC World have also been affected by the strike.
Live coverage of the Chelsea Flower Show could also be hit later in the day, with union members planning to mount a picket line at the venue.
Some stars of BBC radio, such as Radio 1 Breakfast Show presenter Chris Moyles and Radio 2's Terry Wogan, did break the picket line and went to work as normal.
The Today programme, which journalists said was due to be presented this morning by John Humphrys, was cancelled, with an announcer telling listeners: "We are not able to bring you the Today programme because of industrial action."
The normal mix of news and high-profile interviews was replaced at 6am by a repeat of In Business, followed by a repeat of a programme made by Tory MP Kenneth Clarke about jazz music.
The strike is the biggest to hit the corporation for more than a decade.
On BBC television, the normal Breakfast programme presented by Natasha Kaplinsky and Dermot Murnaghan was replaced by a basic service with a single presenter.
After midnight, BBC News 24 and BBC World switched to recorded news bulletins, although they later resumed some live broadcasting.
News programmes will be badly affected by the stoppage.
There was a brief news bulletin on Radio 4 at 6am in place of the usual Today broadcast, which was then followed by a pre-recorded programme.
On Radio Five Live, some of its live news and sport was being replaced by pre-recorded programmes.
The National Union of Journalists, BECTU and Amicus said they expected 11,000 workers to join the walkout.
Picket lines began at midnight at Bush House in central London and BBC TV Centre in Shepherds Bush.
Journalists and technical staff across the UK joined picket lines outside regional BBC offices from 5.30am.
Union leaders said support for the strike had been "rock solid".
Gerry Morrissey, assistant general secretary of BECTU, said: "We are extremely happy with the response from our members and we are confident that programmes will be disrupted throughout the day."
Jeremy Dear, general secretary of the NUJ, said it would be the biggest strike against the BBC in living memory.
"It will be a massive display of anger across the BBC at the scale of the impact of these cuts.
"Workers will give a clear demand that managers should start listening to their concerns," said Mr Dear.
"We have made it clear we will not accept cuts which decimate programmes, devalue the BBC, short-change licence fee payers, increase pressures on staff and worsen working conditions."
The NUJ and BECTU said they had recruited 2,000 new members between them since BBC Director General Mark Thompson announced the job losses in March.
Mike Smallwood, national officer of Amicus said: "The savage cuts proposed will damage programming as well as the organisation and will unravel British broadcasting traditions.
"The BBC is a unifying British institution which acts as the nation's conscience but these redundancies will damage the UK at its core."
The BBC has warned the unions that by taking industrial action they were putting at risk the corporation's relationship with the public.
"Industrial action will not remove the need for further consultation or the need for the BBC to implement changes which will enable us to put more money into improved programmes and services," the BBC has said in a statement.
The BBC vowed to do everything it can to broadcast programmes today.
Mr Thompson sent a message to staff on Friday saying he wanted to return to negotiations with the unions as soon as possible.
He said he recognised that today would present "difficult choices" for those who had voted to strike as well as for those who had not.
A handful of people were picketing BBC's Broadcasting House in central London, with passing drivers hooting their horns in support.
Striking staff had formed picket lines outside all the major entrances to BBC Television Centre in White City, west London.
Union representatives were using hand-held video cameras to film the trickle of workers who ignored signs urging them not to cross the picket line.
Wogan wished people on the picket line "Good luck" as he went inside Broadcasting House this morning, a BECTU representative said.
Richie Ellison, a technical operator, said: "He said 'Sorry I cannot help, but good luck', which we thought was nice.
"Everyone seems quite nice about it so far from the presenters.
"I think they realise they are presenters who have to keep the service going."
Radio 2 presenter Sarah Kennedy and Sheila Fogarty, of Radio 5 Live, also broke the picket line, a union representative said.
But he claimed that Fogarty had been unable to get the show running and a programme about the history of boxing was being aired instead.
Up to 11,000 staff were involved in the first of a series of walkouts in protest at the jobs cuts.
One of the biggest picket lines was outside TV Centre in west London where journalists and technicians were joined by TUC general secretary Brendan Barber and veteran campaigner Tony Benn.
The journalists held up banners which read: "We're on strike to save the BBC!"
Union leaders said they feared the BBC would serve an injunction on them over picketing at the Chelsea Flower Show because technicians would not normally be based at the show.
Tony Lennon, president of the technicians' union BECTU, said: "It is scandalous that because people are working 'on the road' they are not allowed to try to persuade other people to join the strike."
He said the BBC had brought in managers and freelance workers to make sure that televising of the Flower Show went ahead.
Union leaders said the strike had been "rock solid" and would seriously disrupt news, sport and other programmes throughout the day.
Mr Dear said the strike had been an "astonishing success", adding: "We are absolutely delighted with the level of support we have been given which shows the strength of feeling among BBC staff at the scale of these cuts."
The unions have called a 48-hour strike from next Tuesday and have warned of another 24-hour strike next month.
They are urging Mr Thompson to reopen negotiations over the job cuts which the corporation announced in March under plans to make savings of £355 million.