Fifteen people, including a 16-year-old boy, were arrested following clashes with police during an anti-austerity protest close to Downing Street.
Officers in riot gear clashed with a "minority" of protesters who threw objects during the rally - which came after the Tory election victory.
Met Police said four police officers and a police staff member were injured.
A police investigation is also under way after graffiti referring to "Tory scum" was daubed on a war memorial.
Met Police said of the 15 arrested, 14 people been bailed pending further enquiries including a full review of CCTV footage and a 24-year-old man remains in custody after being arrested on suspicion of assault on police.
Graffiti was sprayed on the Women's War Memorial, in Whitehall, close to where the Queen attended a service of thanksgiving at Westminster Abbey to mark the 70th anniversary of VE Day - the end of World War Two in Europe.
A Downing Street spokesman condemned the graffiti as "a despicable display of disrespect for those who fought and died for their country".
Hundreds of anti-government protesters, some holding placards reading "I pledge to resist" and "Stop the cuts", took part in the rally on Saturday, which began outside Conservative Party headquarters.
In a statement, the Met said 12 people were arrested on suspicion of violent disorder and three for assault on police.
Among the objects thrown at police were traffic cones and smoke bombs.
One officer suffered a dislocated shoulder and a police staff member was hit in the mouth by an object, the force said. Both were being treated in hospital.
Three other officers were assaulted but did not go to hospital, police added.
There have been no reported injuries to any protesters.
Ch Supt Gerry Campbell said the "vast majority" of protestors had taken part peacefully, but added that a "small minority" had been intent on causing disorder.
"We have launched an investigation into criminal damage to the Women in World War Two Memorial. This is particularly abhorrent considering we are celebrating the 70th anniversary of VE Day," he said.
One student journalist who was at the scene told the BBC: "There were thousands of people out there to protest against five years of Tory rule, which is going to see more cuts, more privatisation and more austerity - and one person's action should not represent that."
In Cardiff, about 200 people joined a similar rally against austerity - where singer Charlotte Church carried a placard saying she was "mad as hell".