Anti-capitalist protesters said they had "fulfilled" a goal when they moved into Paternoster Square, home to the London Stock Exchange, on tuesday evening.
Activists, who were prevented from occupying the area in October, said 50 people accessed the square with tents and supplies at around 7pm.
An Occupy London spokesman said their successful arrival proved their movement was a "force that will not be stopped".
The site is just yards from St Paul's Cathedral where demonstrators dug in for four-and-a-half months after their first bid to pitch tents on Paternoster Square was derailed by police.
"This was something we wanted to do on 15 October and we have finally done it," the spokesman added.
"Even if we lose it tonight, it proves that we can do it. This is what it's about. This is a force that will not be stopped."
Activists were unable to say how long they expected to remain on the square.
"There's an injunction at Paternoster Square so they are waiting for a while," the spokesman added.
"We don't know what's going to happen but we have fulfilled a medium term goal which is of course rather satisfying."
Earlier, members of the Occupy group handed out thousands of flowers to passengers at London's Liverpool Street station, bearing the message: "There Is Something Better Out There" during a day of rallies and marches across the country.
Their camp outside St Paul's forced it to close for a week in late October for the first time since the Second World War.
It also led to the resignation of the cathedral's Canon Chancellor Dr Giles Fraser and then Dean of St Paul's, the Rt Rev Graeme Knowles.
A hundred protesters milled around close to the Stock Exchange where one activist had set up camp on the Paternoster Square Column.
The campaigner quietly puffed on a cigarette as he sat on his perch next to a Maypole, under a banner reading "Occupy London".
Around 30 or 40 police officers stood guard amid reports that a cherry-picker was being dispatched to remove the man.
Other demonstrators gathered in the square - many wearing white masks associated with hacking group Anonymous - expressed their support for his cause.
A 26-year-old charity worker, who gave his name only as Johan, said: "This is a statement against crony capitalism.
"We started earlier today and by the time we got here, there were no police.
"I may stay the night but I'm not sure yet.
"It's more about the public statement than anything else."
He said the activist on the column had climbed up unassisted earlier in the evening.
"I think they're going to try and get him down with a cherry-picker," he added.
A row of six tents had been set up at one end of the square.
One campaigner said she had come to join tonight's demonstration in "solidarity" with workers.
The woman, who refused to give her name, said she had been living in tents for six months, at St Paul's and on nearby Finsbury Square.
Another demonstrator, who would not give his name, said there was "no particular reason" why he had arrived at tonight's demonstration.
"A friend told me it was on so I came down," he said.
Police later formed a semi-circle around the demonstration and told protesters they had 30 minutes to leave the area.
The six tents were swiftly picked up and removed from their temporary pitches.
Those who remained within the square after the half hour police warning were told they faced arrest if they refused to leave the area immediately.
Activist Tanya Paton, 40, from Halstead in Essex, walked away from the protest some minutes earlier saying protesters were "delighted" to have briefly set up camp in Paternoster Square.
"As long as the government continues to serve the super-rich, we are going to continue to fight and try and change things," she said.
Musician Denis Fernando, 36, from Wembley, north west London, promised further action, adding: "We are the first green shoots of what's going to be a very long spring."
Most demonstrators were cleared from the square at around 10.30pm. A small group remained sitting on the ground surrounded by police.