Hundreds of construction workers have taken part in wildcat strike action in a dispute over pay and conditions.
Protests were staged at sites in London, Cardiff, Manchester, Merseyside, Glasgow, Hull and North East Lincolnshire on Wednesday morning.
Contractors say changes proposed by seven construction companies would mean lower wages, but the firms claim many workers would benefit.
The union Unite said it had not officially backed the strike action.
In a recent ballot union members had voted 80% in favour of strike action, which was originally intended for Wednesday.
However, Balfour Beatty Engineering Services challenged the walkout and a re-ballot was ordered by the trade union.
An estimated 200 workers gathered at the Conoco oil refinery in Immingham, North East Lincolnshire. In Glasgow, protesters stood outside the headquarters of Balfour Beatty Engineering Services.
There were similar scenes at London's Blackfriars station, St Catherine's Hospital in Merseyside, Manchester Central Library, Kelvin Hall School in Hull and a hospital in Cardiff.
Chris Weldon, a regional officer for Unite, said: "I believe the men are extremely frustrated at the moment that they're going to have these terms and conditions foisted upon them, which would be a 30% cut in their livelihood.
"I fully understand why the guys are taking the action.
"We've been warning employers that unless they come back to the table and negotiate with the union then things like this are going to take place."
Blane Judd, chief executive of the Heating and Ventilating Contractors' Association (HVCA), which represents the seven companies, said: "The claims by protesters and the Unite trade union that the new proposed agreement will lead to massive pay cuts and workers heading for the sack are total fiction.
"No-one will take a pay cut - in fact 30% will see an increase in their pay packets - and no-one will lose their jobs."
He said a new national agreement aimed to boost business efficiency, safeguard jobs and ensure pay equality.
Mr Judd said: "The BESNA national agreement is about responding to changes in the construction industry.
"If we don't evolve then we can't remain competitive and we could see British firms lose jobs to foreign contractors."