Seven other people were arrested during the melee for possession of "offensive weapons" -- including bottles of bleach, a hammer and a chisel, police said.
Police said incidents at the scene began when the crowd -- believed to be either members of an anti-fascism group or there to provide support to the Harrow Central Mosque -- went after three youths, "caught up with them, and assaulted them," police said.
"Police had to intervene to prevent the crowd from continuing with their attack."
It was not immediately known if the youths were affiliated with any political group.
Police said they had identified and stopped a number of people believed to be heading to the right-wing protest prior to the demonstration's start time, preventing them from reaching the area.
It was not immediately known how many, if any, of the group's supporters had successfully arrived outside the mosque.
"One person was arrested to prevent a breach of the peace as police believed if the SIOE demonstration started it would have resulted in serious disorder," police said.
The group said on its Web site that the demonstration had been called off and that Stephen Gash, listed as one of the group's contacts, had been arrested "to prevent a breach of the peace."
"If you are on your way to the demo, don't go, it's being called off right now," the post read. "The police can't handle the muslim counter demonstrators [sic]."
The group describes itself on its Web site as an alliance of people "with the single aim of preventing Islam becoming a dominant political force in Europe."