St Paul's Cathedral has closed to visitors for the first time since World War II because of protesters camping on its doorstep, its dean has said.
The decision was taken with a "heavy heart" for health and safety reasons, said the Right Reverend Graeme Knowles.
Anti-capitalist demonstrators from Occupy London Stock Exchange have been in St Paul's Churchyard since Saturday.
The group said they were "disappointed" by the closure but they planned to continue the protest.
Following a meeting of the protesters, one of the group, who gave her name as Lucy, said: "It was felt by everyone that we really wanted to stay and continue with the protest.
"This protest is massive, it affects everybody, everyone's watching at home right now.
"It's not just about a few people who have got some tents in St Paul's, it's not a stunt, it's not a spectacle."
Earlier a statement from Occupy London Stock Exchange said the camp had been reorganised in order to meet fire safety concerns.
The dean has asked the activists to move on from the area by the entrance.
"We have a legal obligation to keep visitors safe and healthy," he said.
Peter, a supporter from Occupy London Stock Exchange, said: "What we are doing is we are informing everybody of the current situation and from then on we will be deciding if any further action will be taken on everyone's behalf.
"We knew this would be a difficult occupation and I don't think that we can just give up at this very moment."
The cathedral was closing to members of the public after a service on Friday afternoon.
On Sunday the cathedral's canon chancellor, the Reverend Giles Fraser, said he had asked for the police presence outside the building to be scaled back.
By Wednesday, the higher number of protesters meant officials had to review "the extent to which it can remain open for the many thousands coming this week as worshippers, visitors and in school parties".
Speaking outside the cathedral on Friday, Dean Knowles said: "I have written an open letter to the protesters this afternoon, advising them that we have no lawful alternative but to close St Paul's Cathedral until further notice.
"With so many stoves and fires and lots of different types of fuel around, there is a clear fire hazard.
"Then there is the public health aspect, which speaks for itself.
"The dangers relate not just to cathedral staff and visitors but are a potential hazard to those encamped."
'No safety issues'
The 200 staff and 100 volunteers, who work at the cathedral, would continue coming to work "as usual", a spokeswoman said.
Sunday services have been suspended until further notice but small gatherings of up to 100 people would still be allowed inside the church to enable planned weddings to go ahead, the cathedral said.
It is the only the second time Sunday services have been cancelled - the other time was during World War II, when the cathedral was closed in 1941 for four days during the Blitz.
Last year the cathedral earned an average of £22,600 a day from commercial activities, which included income from 820,000 paying visitors.
A statement from the protesters said: "Since the beginning of the occupation six days ago, OccupyLSX have tried hard to accommodate the cathedral's concerns in any way we can.
"Over the past 48 hours, we have completely reorganised the camp in response to feedback from the fire brigade and we have also accepted the presence of two large barriers to preserve access to the side door of the Cathedral.
"This afternoon we have been told, in a telephone call, by the fire brigade, that they have not issued any new requirements above and beyond those already communicated directly to the camp. Therefore, there are no outstanding fire safety issues."
The statement called on the cathedral to specify its "precise safety concerns", saying the closure of the restaurant "mystified" them as the access to it had "never been blocked by the encampment".
City of London Corporation's policy and resources committee chairman, Stuart Fraser, said: "We hope common sense will prevail and those camping around the cathedral will recognise that they are damaging the integrity of their protest by their actions - and they decide to disband in a peaceful manner."