The government was accused tonight of giving itself draconian powers to clamp down on protests at the 2012 Olympics. Critics said the powers were so broad they would potentially give private contractors the right to forcibly enter people's homes and seize materials.

Opposition parties and civil liberties groups criticised the powers as top security officials announced plans concerned with keeping the games, to be held mostly in London, safe from terrorist attack and from "domestic extremists" and public order problems like disruptive protests.

The legislation is directed at curbing advertising near the Olympic venues. A government spokesperson said the laws, passed in 2006, were meant to stop "over-commercialisation" of the games.

But civil rights campaigners are worried about several clauses in the London Olympic Games and Games Act 2006. Section 19(4) could cover protest placards, they said, as it read: "The regulations may apply in respect of advertising of any kind including in particular – (a) advertising of a non-commercial nature, and (b) announcements or notices of any kind."

Section 22 allows a "constable or enforcement officer" to "enter land or premises" where they believe such an advert is being shown or produced. It allows for materials to be destroyed, and for the use of "reasonable force". The power to force entry requires a court warrant. Causing still further concern is a section granting the powers to an enforcement officer appointed by Olympic Delivery Authority.

Anita Coles, policy officer for Liberty, said: "This goes much further than protecting the Olympic logo for commercial use. Regulations could ban signs urging boycotts of sponsors with sweat shops. Then private contractors designated by the Olympic authority could enter homes and other premises in the vicinity, seizing or destroying private property."

The Liberal Democrats' home affairs spokesman, Chris Huhne, said: "This sort of police action runs the risk of using a sledgehammer to crack a nut. The police should take a deep breath and read the excellent report from the chief inspector of constabulary on the tolerance of protest. We should aim to show the Chinese that you can run a successful Olympics without cracking down on protesters and free speech."

Chris Grayling, the shadow home secretary, said: "This is a government who just doesn't understand civil liberties – they may claim these powers won't be used but the frank truth is no one will believe them. Neither the police nor any other official should be invading people's homes for what appear to be commercial reasons."

A senior government security official said the powers would not be used to suppress protests or political placards. And the assistant commissioner, Chris Alison, in charge of the policing of the 2012 Olympics, said: "We are not going into people's houses to stop people protesting."

But Peter McNeil, who opposes the staging of equestrian events in Greenwich park, told BBC London: "This is dreadful. It's bullying taken to another level."

A spokesperson for the Department for Culture, Media and Sport said the legal provisions on games advertising were meant to "prevent ambush marketing – not prevent or restrict lawful protests".