Striking refuse and street cleaning workers in Leeds have been accused of intimidating non-striking staff outside household waste sites.
The council said people arriving for work at one depot had their cars surrounded by up to 20 union members.
It also said residents had reported being obstructed by pickets as they tried to drop off their recycling.
The Unison union said picket lines could get "heated" but denied that any public disorder had taken place.
Employees are picketing refuse depots in the city for the third day in protest against plans by the council which it is claimed could lead to pay cuts of up to £6,000.
Leeds City Council's executive board member for environmental services, councillor James Monaghan, was confronted by striking refuse workers when he visited the Stanley Road site on Wednesday.
'Dirty tricks'
A council spokesman said staff leaving or entering waste-sorting sites had reported being "verbally abused" by those on the picket line.
And he said the opening of the Stanley Road site was disrupted on Tuesday "as the site locks had been super-glued".
Joint council leader Richard Brett said: "They are quite serious allegations. Some of the things that are alleged to have taken place we regard as totally unacceptable.
Unions say hundreds of workers are involved in the strike
"The sorts of things that I am told have happened... that's not the sort of thing that is actually going to win friends for the unions.
"Their case is that they're the aggrieved party and they weaken that case hugely if they get up to dirty tricks."
Tony Pearson, from Unison, denied the council's allegations and said police officers were at the picket lines to prevent any acts of public disorder.
He said members of "one or two" pickets had approached workers' cars "under police supervision" and had had "quiet conversations with people".
"In relation to vandalism obviously no union could condone vandalism," Mr Pearson said.
"But I have to say the police haven't reported any incidents of vandalism to us, nor have the council."
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