Greater Manchester Police say they are determined to find those responsible for disorder at a Tommy Robinson political campaign event in Oldham at the weekend.
Footage captured by supporters of Mr Robinson, who is standing as an MEP candidate in north west England, shows clashes with counter-demonstrators during which two police vehicles were damaged and eggs and bricks thrown.
Video posted to social media shows a group who self-identify as the Muslim Defence League shouting across a police blockade throwing missiles and shouting "racist scum".
Jim McMahon, Labour MP for Oldham West and Royton, tweeted: "It's horrible to see our town being used for hate and division when our own future is a shared one, long after the circus has left town."
Mr Robinson is standing alongside candidates from Change UK, the Conservatives, Green Party, Labour Party, Liberal Democrats, UKIP, Brexit Party, UK EU Party, English Democrats and one other independent candidate Mohammad Aslam in the region.
Police said no-one was seriously injured in the disturbance, and reports someone had been stabbed were false.
Chief Superintendent Neil Evans said speculation was "unhelpful", but urged anyone with footage to come forward.
In a statement released on Sunday, he said: "For the small few who ruin this for everyone else, let me be clear; you can expect to be identified, investigated by us and brought in for questioning.
"Everyone should be able to exercise their right to free speech but when people hide behind this to hurl objects and abuse and act in such a disgusting way, there simply is no excuse.
"Since last night, we've been working alongside our partners and people in the community and will continue to do so over the coming days. I understand that people will still have concerns around this and while our investigation continues, officers will be patrolling the area so feel free to approach them with any concerns you may have."
Voting in the election takes place on Thursday, while counting starts on May 26.
Police have appealed for anyone with information about the disorder to contact them and urged people with footage or images to send them to firstname.lastname@example.org.
To see the full list of MEP candidates, click here.