The government called in mediators from Acas last night in an urgent attempt to end the dispute over the exclusion of British workers from construction contracts which led to a wave of wildcat strikes across the country yesterday.

The attempt to halt the protest came as it emerged that hundreds more workers were ready to join the nationwide strike action. Bosses at Sellafield nuclear power plant confirmed that 900 contractors will vote on a walkout on Monday morning in solidarity with oil industry contractors in Lincolnshire which is at the heart of the dispute after around 300 new jobs were taken by Italian and Portuguese workers.

The conciliation service was called in after around 3,000 workers at oil and power plants across the UK staged unofficial strikes in support of workers at the Lindsey refinery at North Killingholme. The TUC claimed the refinery owner, Total, had made an "apparent attempt to undercut the wages, conditions and union representation of existing staff".

In angry scenes outside plants from Scotland to south Wales, union leaders spoke out against European workers taking construction jobs in the oil and energy industry ahead of British workers. Outside the Lindsey refinery, some protesters called on their colleagues to march on Downing Street.

Shop steward Kenny Ward told the crowd they had to stand together and take on the "greedy employer". He said: "I'm a victim, you are a victim, there are thousands in this country that are victims to this discrimination, this victimisation of the British worker."

Total had put a contract out to tender for a £200m construction project with five UK firms and two European contractors. The Italian company IREM won the contract and supplied its own permanent workforce. It is understood 100 Italian and Portuguese workers are already on site and 300 more are expected next month.

In apparently co-ordinated action, 700 workers at the Grangemouth oil refinery near Falkirk walked out, and 400 more downed tools at the Wilton chemical site in Cleveland. Early morning protests flared at at least eight other facilities in Scotland, Wales and Northern Ireland.

In Downing Street and Davos, where Gordon Brown had been speaking against protectionism at the World Economic Forum, the government came under pressure to respond to the crisis. There was criticism that the award of the contract at the Lindsey refinery appeared to cast into doubt the prime minister's pledge in 2007 to deliver "British jobs for British people". "I understand people's worries about their jobs," Brown said.

A Downing Street spokesman said the contract taken by the foreign workers had been agreed "some time ago when there was a shortage of skilled labour in the construction sector in the UK".

Pat McFadden, minister for employment relations, said he understood the strikers' concern about employment but could not condone their unofficial walkouts.

"What is important in a sensitive situation where tempers are running high is to take an independent and dispassionate look at what is going on," he said.

Union leaders were furious that with unemployment rates soaring, British workers had been overlooked. The TUC accused employers of attempting to undercut the wages, conditions and union representation of existing staff.

Bobby Buirds, a regional officer for the union Unite in Scotland, said: "The argument is not against foreign workers, it's against foreign companies discriminating against British labour. This is a fight for work. It is a fight for the right to work in our own country. It is not a racist argument at all."

Nevertheless, there were indications that far right politicians were eager to seize on the dispute. The British National party, which is hoping to make a breakthrough in this year's European parliament elections, said it had sent a team of supporters to join the 800 workers gathered outside the north Lincolnshire plant.

Brendan Barber, secretary general of the TUC, said workers were "rightly angry" at employers who have not given British based workers the opportunity to apply for new jobs but added: "The anger should be directed at employers, not the Italian workers."