Protesters attempting to highlight human rights violations clashed with pro-China supporters during a procession welcoming the Chinese president to the UK.
Scuffles broke out between the two groups after the police perimeter set up for protesters was ignored by both sides.
Thousands of supporters lined The Mall to welcome Xi Jinping with flags, T-shirts and hats - provided by the Chinese embassy, according to one Pro-China organiser.
Around 100 members from various human rights groups attended the procession.
Protesters from Free Tibet group and Pro-China supporters were seen pushing and shoving as demonstrators were placed together as the procession drew closer, with traditional Chinese drums drowning out the demonstrators' chants.
Police scrambled to extend fencing for the protesters, but some opted to remain in the mixed zone after the new perimeter was moved two metres back behind three fences.
Free Tibet campaign organiser Alistair Currie said this broke an agreement made with police that they would not hold back protesters from The Mall edge.
He said: "The original plan was for there to be a pen for the Chinese protesters and a pen for us. But the Chinese flooded the place and police didn't hold them in. As a result we had little room."
While it was not violent, scuffling from both sides meant police had to intervene.
Mike Beavis and his wife Carol, members of Free Tibet, chose to stay penned in with pro-China supporters because they were closer to the procession.
Mr Beavis said: "They were shouting and trying to stop me waving my flag by putting theirs in front of mine. Not only did they have Chinese flags but they had Union jacks - so they were trying to use my flag against me.
"The powers-that-be have pandered to the Chinese and stopped our group from being at the front."
One 81-year-old Free Tibet protester was threatened with arrest after failing to move when police set up the new perimeter.
Refusing to give her name, she said she "couldn't believe this is Britain".
Amnesty UK also attended the protest but chose not to go in the designated pen, because it looked "constraining".
Talon Li, a Chinese finance student at Greenwich University, set up his flag at 5.30am.
The pro-China demonstrator said: "UK and China will really help each other. They should stay friends - every British and Chinese person can be friends."
Protesters from Free Tibet booed the procession as it passed, meanwhile, pro-China supporters played the Chinese national anthem through a megaphone.
Earlier Amnesty UK's photo call criticising China's human rights record was interrupted by pro-Chinese supporters.
Security is tight for the visit and police have spent over five months planning for it.
Police said no arrests had been made.