Big Ben falls silent for repairs
Big Ben has fallen silent for major repair work expected to last until 2021.
The midday bongs were the last regular chimes from the famous bell until the repairs to its tower are complete.
It will still be used for special occasions, including New Year's Eve and Remembrance Sunday.
There were cheers and applause from a crowd of tourists and onlookers on the green opposite as the final chime rang out.
The decision to switch it off to protect workers' hearing has sparked a debate about the length of time it will be silent, with MPs not being told it would be four years, the longest period in its history.
Prime Minister Theresa May has said "it can't be right" that the famous bongs will not be heard again until 2021 and has asked for the proposals to be reviewed.
The House of Commons has said it will look again at the length of time Big Ben will be silenced after "concerns" were raised.
The 13.7-tonne Great Bell has sounded on the hour for 157 years and last fell silent in 2007. The Great Clock it forms part of is to be dismantled and repaired.
The BBC's Tom Moseley in Parliament Square
Almost as one, the crowd that had gathered in Parliament Square and on the pavement outside the Palace of Westminster pointed their mobile phones up towards the famous clock.
The passing traffic paused, and as the last of Big Ben's bongs sounded - at least until November - cheers broke out.
Most of the people who had taken up Parliament's suggestion to witness the last bells spoke of their sadness at the switch-off, and plenty, like Brian Mignot from Tasmania, questioned the four-year timescale.
Some, like engineer Neil Tomlinson from Derbyshire, took a pragmatic approach.
"It's got to be repaired, so they're repairing it," he said, bemused at the fuss the plans have caused.
Others just said they wanted to be there.
"It's a momentous occasion", said one of the onlookers, Annette Nicholson, from South East London.
"I thought the crowd would be even bigger."
Mark Tan, who had just arrived in the UK on holiday from Singapore, said: "It's a historic moment but it's good so that it will continue for another 100 years".
Inside the Parliamentary estate, there was more clapping and cheering as noon was struck.
Labour MP Stephen Pound - who had hoped to be joined by "like-minded traditionalists" to witness the last bongs, said it was a "desperately sad" moment and that the decision showed a "real poverty of imagination".
Conservative Peter Bone said MPs had not been told the length of the switch off, adding that the timescale "doesn't make any sense".
"Maybe Big Ben will be silenced for a little while, but perhaps just until the end of this year, and then back bonging away from the New Year," he said.
But the SNP's Pete Wishart told them to "get a grip" over their "bizarre obsession" with the issue.
"The fact that even the prime minister is wasting time on this non-issue shows just how out of touch the Westminster bubble is, when MPs should be working for their constituents on the issues that really matter," he added.
Liberal Democrat MP Tom Brake, a member of the House of Commons Commission, said one concession to critics could be allowing Big Ben to chime on special occasions.
But he said it was "too early to say" whether it would be used on the day the UK leaves the EU in March 2019, as demanded by some Eurosceptic Conservative MPs.
He added: "I understand why people would get excited about Big Ben and the Elizabeth Tower and the clock because they are so symbolic and people come from all over the world to have their photographs taken in front of it, and that's why we need to ensure that these works happen to ensure that this symbol of our Parliament, our nation, is available for people to relish for years to come."
Straight after Big Ben had finished at midday, nearby Westminster Abbey rang its chimes "to say farewell".
During the repair work, the roof of the Elizabeth Tower will be stripped off and restored, the bell frame repaired, leaks into the clock room stemmed and a lift installed.
A brick enclosure in the tower will also be replaced with glass to allow Big Ben to be viewed by people walking up the staircase.
The "drab" colour scheme on the Great Clock will also be changed to give it a more "vibrant" look, officials have said.
The Ayrton Light, which shines when the Commons and Lords are sitting, will be "off for some time" but the timescale will not be finalised until later this year.