George Galloway's Respect Party has claimed a major scalp in Bradford by ousting the leader of the council.
Labour's Ian Greenwood lost his Little Horton seat, after three re-counts, to Respect's Alyas Karmani.
Mr Galloway, who became Bradford West MP in a by-election victory in March, said: "We took the head off the rotten fish that is Bradford City Council."
Meanwhile, voters in Leeds, Bradford and Wakefield have rejected the idea of introducing a directly-elected mayor.
Respect won five of the 12 council seats it fought in Bradford. Labour saw the number of its councillors rise from 43 to 45.
Mr Galloway was not at the count at the Richard Dunn Sport Centre, saying he was boycotting it after a dispute with the council over passes for his party.
Speaking at his campaign HQ, he said: "By anyone's calculation it's been a fantastic night for Respect.
"We took seats off all three of the mainstream parties.
"We defeated a council leader who sat there, apparently impregnable and utterly complacent, for a decade and a half or more.
"I think that the Labour Party in Bradford will be taken into special measures by the Labour national headquarters."
Mr Greenwood, who had been a councillor in the Little Horton ward for 17 years, said he was "disappointed" at losing his seat, which he lost by 17 votes.
He said he would be "very surprised" if Labour made a deal with Respect to run the council and believed the party would talk to other groups first.
If Mr Greenwood had won his seat it would have seen his party take overall control of the council.
Before the result, no party had overall control in Bradford and Labour ran the authority as a minority administration.
On the night, Labour increased its councillors from 43 to 45, meaning it will need the votes of one other member to give it a majority.
Speaking after his defeat, Mr Greenwood said he thought the key to Respect's success was Mr Galloway coming to the city and energising a mass of youngsters.
"He's made promises that I hope that he can keep but I doubt that he can," he said.
Mr Greenwood said he feared the enthusiasm surrounding Mr Galloway would not last and Bradford would be left with a "generation of disenfranchised and alienated young people".
Voters in Leeds rejected the idea of a directly-elected mayor by a sizeable majority, with 63.3% of voters saying no while 36.7% voted in favour of the idea.
In Bradford 53.2% of voters were against an elected mayor, while 43.3% said yes.
In Wakefield, 62.2% of voters rejected the idea of an elected mayor, while 37.8% were in favour.
Also in Wakefield, the Labour Party retained control of the city's council, gaining 12 seats from the Conservatives, who lost nine, and three from independent councillors.
Labour also held on to power in Leeds, where they gained eight seats - from the Lib Dems, who lost six, and the Conservatives, who lost two.
Meanwhile, Kirklees Council, which covers the Batley, Dewsbury, Huddersfield and Holmfirth areas, and Calderdale Council, which covers the Brighouse, Elland, Halifax, Hebden Bridge, Sowerby Bridge and Todmorden areas, remain under no overall control.
Labour increased the number of councillors in Kirklees from 27 to 32 and the Conservatives lost three seats to leave them with 18 councillors. The Liberal Democrat presence on the council fell from 14 to 10.
In Calderdale, Labour have 20 seats after gaining seven. The Conservatives have 17 seats after losing four, while the Lib Dems lost one and now have 12. Independent councillors lost two seats and now have two.
· All the latest election results are available at bbc.co.uk/vote2012