Scotland Yard said three arrests had been made during the protest and counter-protest outside Downing Street.
Members of the far-right English Defence League (EDL) congregated after their march in Leicester Square, central London.
As one youth was taken away by police, the crowd began throwing bottles at them.
One officer was hit on the head with a glass bottle and the mob followed the officers, chanting "who the f*** is Allah?" before being pushed back towards the square.
Skirmishes broke out around the square, with several men threatening a youth wearing a Palestinian scarf with a stick before police intervened.