Models wearing chains, stockings and gags have been led around Westminster in protest at laws to make owning "extreme pornography" illegal.
From next year, possession of images such as those showing a threat to life or serious injury to a person's genitals will be banned.
Demonstrators opposite Parliament described this as the government interfering with people's sex lives.
Ministers argue the law is needed to cope with more use of internet images.
Traffic hold-up
The demonstration, organised by the Consenting Adult Action Network, was led by photographer Ben Westwood, son of fashion designer Dame Vivienne Westwood.
He paraded two "slaves" - models called Jade and Dolly Blowup - across the road from Westminster underground station and around Parliament Square, with police having to hold up the traffic.
"I think that people might be worrying that what they have got in their video collection might be breaking the law"
A group of about 20 marchers carried placards with messages including "No to thought crime", "Penalise crime, not sex" and "Depiction harms no-one".
Mr Westwood said to the BBC: "Why are the government doing this? I think they are just mucking about.
"They want to seem as though they are doing something to help society, that they must seem strong on law and order.
"Coming from a government that lied about going into war in Iraq, that seems strange.
"There are more important issues to be debated than this."
'Worrying'
Tourists, drivers and workers in suits looked bemused as the models were led around Parliament Square, passing by statues of Nelson Mandela and several past British prime ministers.
Mr Westwood said: "I think that people might be worrying that what they have got in their video collection might be breaking the law. People are going to get a bit nervous.
"I hope our demonstration does change some minds."
The Criminal Justice Act, which was passed earlier this year, shifts criminal responsibility from the producers "of violent and extreme pornography" to consumers.
This, the government says, is necessary to deal with material obtained via the internet from websites based abroad.
'Unfair'
The maximum sentence for possession will be three years in prison.
Campaigners say the new law risks criminalising thousands of people who use violent pornographic images as part of consensual sexual relationships.
Bruce Argue, of the group Esinem, said: "We want to draw attention to what is an unfair and ill-thought-out law."
The Ministry of Justice spokeswoman said: "We believe that the new legislation will only catch material which is already illegal to publish in the UK under the Obscene Publications Act 1959 and therefore material which is already legally available should not be affected."
The act comes into force on 1 January.
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