Minor scuffles broke out after a peaceful start to the Million Mask March in central London.
Thousands of anti-capitalist campaigners descended on Parliament Square wearing the trademark Guido masks synonymous with the Anonymous hacking group.
There was a sudden surge of activity about half an hour into the event, as several people at the front broke off from the agreed route and ran towards the Churchill War Rooms.
They tried to enter Great George Street, a short distance from Conservative Party headquarters, which was closed off.
A line of police and mounted officers formed a barrier against the densely packed protesters, some screaming about inequality. Those who could get their arms above their waist held phones aloft.
A couple of stray bottles were thrust over the barriers towards officers, which was met with cheers and chants of "Shame on you, we're human too".
Terry Small, 20, from Plymouth, emerged from the crowd of protesters in Great George Street with blood pouring from his head.
He alleged he had been hit a number of times by an officer with a baton.
"I was at the front, I was chatting with the officers asking them what their agenda was and why they were trying to stop us walking down the street when it's supposed to be public land," he said.
"I got pushed in the back by people trying to move forward. I get pushed in the back and put my arm up because I know the baton is coming.
"I couldn't move - it took all my force to shoulder-barge my way out of there."
Extra officers were brought in to deal with the protest, which began in Trafalgar Square before winding down towards Whitehall.
A smoke bomb was set off at the top of The Mall later on in the march.
Police said they made three arrests in Trafalgar Square earlier today after noticing a group of men acting suspiciously at around 2.25pm.
A stop and search found they had a range of items including knives, smoke and paint grenades, gas canisters and lock picks.
The three men, aged 38, 50 and 55, were arrested on suspicion of being in possession of offensive weapons and taken to a central London police station, where they remain in custody.
Many protesters carried placards bearing the message "One solution: Revolution", although other pockets screamed "Whose streets? Our streets" as confused tourists looked on and took pictures.
One activist, who gave her name as Lola, said she had travelled from Hull to the capital to speak up for others.
She said: "There are people who aren't represented, they should be here. I am the people.
"The media twist it. We are peaceful, we are doing this in a peaceful manner, but the media focus on the 1% who cause trouble."
Self-proclaimed anarchist Aztecarna Peatonito, from Mexico, said he wanted a peaceful way of spreading a message against inequality.
He said: "We tried for six years to get the press to take us seriously. It is only when we wore the masks that we started getting attention.
"Anyone who is violent is not with us."
Earlier in the parade, 24-year-old Dylan Connolly, from Stoke on Trent, acted as a pallbearer, carrying a coffin inscribed with the words "fear and anxiety".
He said the prop was an emblem of his decision to ignore media messages blaming society's problems on minorities.