The walkout by around 4,000 members of the National Union of Journalists (NUJ) began at 0000 GMT and was reportedly due to cause major disruption, with TV and radio shows enjoyed by millions pulled entirely or cut back.
Most BBC radio and television news programmes will either not go out or be severely disrupted and the British broadcaster's rolling news channel will air live news bulletins only intermittently, reports said.
Picket lines will be mounted at BBC offices across the country in the first of two strikes planned for this month, with the second due to take place on November 15-16.
Negotiations about proposed pension changes between BBC journalists and management broke down last month, with 70 percent of the NUJ's members voting against the broadcaster's latest offer on pensions.
At the heart of the dispute are the BBC's plans to reduce a pensions deficit of 1.5 billion pounds (2.4 billion dollars, 1.7 billion euros) by capping increases in pensionable pay.
"NUJ members across the BBC have consistently dubbed the proposals a pensions robbery," said NUJ general secretary Jeremy Dear.
"That hasn't changed. The BBC have now left members with no choice but to take action to defend their pensions."
In a message to staff, BBC chief Mark Thompson insisted the offer made to journalists was a "fair one."
The industrial action "may manage to take some output off the air or lower its quality," he said. "But strikes aren't going to reduce the pension deficit or make the need for radical pension reform go away."
Popular shows including radio news broadcasts "Today" and "PM" would be taken off air because of the action, while current affairs TV programme "Newsnight" would not be broadcast, The Times newspaper reported.
BBC Journalists Strike
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