Unions said British staff should be doing the work at the oil refinery.
Total, owners of the Lindsey refinery at North Killingholme, said there would be "no direct redundancies" as a result.
At Aberthaw, South Wales Police said officers were at the scene to monitor the protest.
Mr Thomas, of the Unite unions, said the protest was not a "witch hunt" against foreign workers.
But he said there were concerns that future jobs in the industry would not go to British workers.
"What we're concerned about is [we are] in a recession, we're concerned that all the people that have been made redundant in the UK over these issues could be trained and put back into these particular vacancies," he said.
"There's plenty of unemployed construction workers and other workers who can be put in these vacancies."
He added: "We're fighting for our particular jobs for UK construction workers and for the younger generation.
"All we are saying is British jobs for British people."
Around 50 workers held a demonstration at Aberthaw
He said the demonstration was not an official strike and that the protestors were not stopping vehicles going in and out of Aberthaw.
But some workers turning up at the site and joined them, he added.
Alex Fisher, 52, who works as a plater at Aberthaw, said he was protesting to ensure his children could follow him into the industry.
"There's no apprenticeships for them to do anything," he said.
Speaking about a new fuel gas desulphurisation (FGD) plant, which is currently being built at Aberthaw to ensure the power station keeps running, he said: "There's been a smattering of apprenticeships but it could have carried a lot more. Surely you have to train the youth, that's how you cover your skills shortage."
Andy Summers, 60, said: "We have all travelled all over the UK on contracts. I've been doing it for 45 years. All of a sudden we can't get the jobs because of the influx of European labour."
RWE npower, which runs the power station, said: "Around 50 contract workers, who work on either construction or maintenance projects, are peacefully demonstrating at the main entrance of Aberthaw Power Station.
"Police are on site and have sectioned off an area for the demonstration.
"No RWE npower staff are involved and Aberthaw Power Station continues to operate normally."
A South Wales Police spokeswoman said: "A number of protesters have gathered on the roadside outside Aberthaw power station officers are at the scene.
"Our first priority is the safety of the public and the protesters at the site. We are in communication with the protesters and are facilitating a lawful protest."
Workers at two LNG plants are also understood to be demonstrating in support of workers at Lindsay.
Brendon Stack, GMB steward, said workers walked out of the sites on Friday morning after a site meeting with stewards.
He said they had gone home in a one-day protest but will return on Sunday and Monday for their shifts as normal.
A spokesman for South Hook LNG said: "I confirm that a number of contractor workers at the South Hook site have left the site today in support of industrial action being taken elsewhere in the UK.
"The majority of the workforce continues to work normally."
Humberside Police said numbers of protesters at the Lindsey refinery demonstration on Thursday, which passed off peacefully, was near 800.
Employees at BP's Dimlington gas terminal in East Yorkshire and its chemical manufacturing plant in Saltend, Hull, also walked out in support of the Lindsey refinery workers.
Unofficial strike action was also taken by workers at Scottish Power's Longannet power station in Fife.
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