At least 2,000 people are protesting in two political demonstrations in Manchester city centre.
About 700 members of the English Defence League (EDL) and 1,400 members of Unite Against Fascism (UAF) are separated by a line of riot police.
The EDL members are protesting against Islamic extremists - prompting a counter-protest from the UAF.
Police said 34 people had been arrested in Manchester on suspicion of public order offences.
One arrest was also on suspicion of carrying an offensive weapon.
Both sets of demonstrators are facing each other in Piccadilly Gardens and are separated by a line of police officers, dogs and mounted police.
Police officers arrest one of the 18 taken into custodyMat Trewern, from BBC Radio Manchester, said the atmosphere had turned "quite nasty" compared to earlier on in the day.
He said: "The police are trying to keep the two groups separate and it has been working so far.
"Some members of the UAF tried to break the police line between the two groups, which in turn angered the EDL members.
"Trouble had started when 100 members of EDL arrived at Piccadilly Gardens and they were immediately met with shouts of 'racists' and 'off our streets' by members of the UAF, who had already congregated at Piccadilly.
"The atmosphere is tense."
'Hell-bent on confrontation'
He said the number of protesters from the UAF outnumber those from the EDL by about two to one.
Assistant Chief Constable Garry Shewan said: "The presence of so many protesters in the city has proved a challenge and while many have turned out to protest peacefully, the police reaction has been necessary in order to prevent the few hell-bent on violent confrontation.
Protesters are being separated by rows of police officers"Today we've made arrests, many of whom were thought to be agitators and trouble-makers."
Greater Manchester Police confirmed one man, believed to be heading to the protest, had been arrested in Birmingham on suspicion of distributing racially aggravated material.
Muslim leaders had renewed appeals for people to avoid the demonstrations.
A recent EDL event in Birmingham led to counter-demonstrations and bricks being hurled at riot police. Up to 90 people were arrested.
Nanu Miah, a community leader from Oldham, said: "We are not encouraging people to go, we don't know who EDL is and what could happen."