Police were called as striking workers picketing outside a Sheffield food factory prevented a lorry leaving the site.
Workers at Pennine on Foods Drake House Crescent, Waterthorpe, started a 48-hour strike on Sunday morning.
They say the company is changing their terms and conditions following the introduction of the National Living Wage which will mean cuts to Bank Holiday and weekend working rates – with some staff losing up to £5,000 a year.
Workers mounted a picket line outside the factory on Sunday but police were called when they stopped a delivery lorry leaving.
Haroon Rashid, from the Bakers, Food and Allied Workers Union, said the incident passed peacefully – but that staff were angry.
He said: “There was an altercation on site and police were called.
“Our members were trying to prevent a lorry from leaving but it passed off peacefully.”
He said more workers would be on the picket line on Monday.
Pennine Foods makes around 750,000 ready meals a week for Marks & Spencer.