Police said up to 400 demonstrators gathered at Wilton
Up to 600 demonstrators gathered outside chemical and steel plants on Teesside in support of striking refinery workers in Lincolnshire.
The move followed the second day of protests at the Lindsey Oil Refinery in North Lincolnshire over a £200m contract given to an Italian firm.
Cleveland Police said at least 400 people were outside the gates of the former ICI complex at Wilton.
A further 200 were outside the Corus steel mill near Redcar.
Unions said the Lindsey contract should have been given to British workers.
Demonstrators on Teesside gathered from about 0600 GMT and remained at both sites for about three hours.
George Mitten, of the union Unite, said the protests were peaceful and were aimed at preserving UK jobs during the recession.
The protests caused huge traffic tailbacks, with many roads around the Wilton and Corus sites gridlocked, according to the police and the AA.
A spokeswoman for Cleveland Police said there had been no reports of arrests.
Mr Mitten, who was with demonstrators at Wilton, said: "Foreign companies are coming in and undercutting British firms, yet not paying agreed rates of pay.
"All we are doing is showing our support to those in Lincolnshire and trying to highlight this issue to the government. Gordon Brown has said 'British jobs for British workers' but he has not backed it up.
"Feelings across the country are running high at the moment because a lot simply feel in the not-to-distant future we simply won't have a job."
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