A 17-year-old arrested after this week's student fees protest has been released on police bail.
The teenager was the 57th person to be questioned in connection with Wednesday's riot during which a fire extinguisher was launched from the roof of Millbank Tower after protesters stormed the Conservative Party headquarters.
A student at Anglia Ruskin University, Cambridge, has already been questioned in connection with the incident and released on bail. The 23-year-old, originally from Reading, Berkshire, was held at Parkside police station in Cambridge.
A Met spokesman said: "A 17-year-old male arrested on suspicion of violent disorder has been bailed to a date in mid-February pending further inquiries."
Police Federation representatives have called for the person who flung the empty metal fire extinguisher from Millbank Tower to be charged with attempted murder.
Senior officers said it narrowly missed injuring two territorial support group officers, brushing down the back of one and hitting the knees of another.
The incident was one of the defining moments of the four-hour stand-off after a breakaway group of students attacked the Millbank office complex.
Up to 50 demonstrators smashed windows, discharged fire extinguishers and threw debris from the roof of the building.
But hundreds of others gathered in a forecourt below booed the culprit and shouted to others to stop damaging the building and throwing missiles.
The Met said 10 of the 56 people arrested during sprawling outbreaks of disorder were under 18. A spokesman said most of the others were students and aged between 18 and 26.