The Metropolitan Police struck the "wrong" balance between cracking down on violence and allowing the right to protest during Wednesday's student fees demonstration, Police Minister Nick Herbert has said.
Policing of the protest "clearly did not go to plan", Mr Herbert said, vowing the force would "learn the lessons" from the scenes which saw demonstrators storm the building housing Conservative Party HQ and led to 50 arrests.
But he said the blame for the "appalling" violence lay solely with the perpetrators.
Shadow home secretary Ed Balls agreed, describing yesterday's vandalism as "completely unacceptable" and carried out by "thugs".
In a Commons statement to update MPs on the protest, in which 41 police officers were injured, Mr Herbert said: "The police have to strike a balance between dealing promptly and robustly with violence and unlawful activity on the one hand, and allowing the right to protest on the other.
"Clearly, in this case, the balance was wrong. But these are difficult decisions and they are not taken lightly."
A probe had been launched by the Met to examine why the number of demonstrators and the scale of violence had not been anticipated, Mr Herbert said.
And he went on: "Wednesday's protest and the policing clearly did not go to plan."