Activists from Unite, Britain's biggest union, and NIPSA, which represents thousands of public sector workers in Northern Ireland, said measures had been put in place to prevent demonstrations in Belfast and Fermanagh being hijacked by troublemakers.
In London, Scotland Yard continued to urge activist groups to contact them, but said that so far none of the protesters have done so.
The force said a protest is expected to rally at 2pm in Carlton Gardens near Pall Mall, and that officers will be deployed there and in key areas around Oxford Street and in Soho.
Scotland Yard said in a statement: "Officers will be deployed at key locations and also act as a flexible reserve across London that can be quickly moved to respond to any incidents.
"We continue to ask protesters to contact us and work with us. What we will continue to do is uphold people's right to protest peacefully, whilst balancing the rights of others to go about their daily business, plus uphold our duty to prevent crime and disorder.
"There is information to suggest that there will be further protests in London today. As yet no one has chosen to work with us."
Ed Argar, from Westminster City Council, said protesters did not have a licence to cause disruption.
He said: "Yesterday's events demonstrate that, as ever, there were a small number of people intent on turning a legitimate protest into confrontation.
"I hope any further protests around G8 pass off peacefully. We fully support the right to legitimate protest, but that is not a licence to disrupt the daily lives of residents, visitors and businesses or the right to break the law."
Yesterday demonstrators occupied a former police building in Beak Street, Soho, before officers dramatically forced their way in.
Television footage showed climbing teams securing the roof, and bundling one protester on to his front after he ran on to the flat rooftop. The building is clear today but officers remain on guard at the site.
In total 57 people were arrested for various offences including violent disorder, possession of pointed or bladed articles and a variety of public order offences, and around 1,200 officers were deployed.
The demonstration saw protesters surge down Piccadilly, but they were blocked from accessing the historic Fortnum & Mason department store which was previously targeted during anti-cuts protests in 2011.
Around two dozen gathered near BP headquarters in St James's Square amid a heavy police presence, while others moved around Oxford Street.
A building near Liverpool Street was also raided and three arrests were made.
Police said there was intelligence to suggest that paint bombs and shields could be used against horses and officers in Westminster and the City of London.
Powers to stop and search in anticipation of violence and to require the removal of disguises have been authorised.
At next week's G8 summit, to be held in Northern Ireland, the leaders of the world's eight wealthiest countries, including Russian President Vladimir Putin and German Chancellor Angela Merkel, are due to meet at the luxury Lough Erne resort in Co Fermanagh.
US President Barack Obama is also due to visit parts of Belfast ahead of the summit.
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