"Participation in the strike is very high, production in the factory is at a complete stop," said Eduardo Florindo, head of the SITE Sul union, one of two which called the 24-hour strike.
The assembly facility with 3,500 employees has never gone on strike in its over 20 years of operation, except during general strikes in the eurozone nation.
The introduction of the Saturday shift at the Autoeuropa plant in Palmela, about 30 kilometres (20 miles) south of the capital Lisbon, will allow for production of the new T-Roc compact SUV that Volkswagen plans to have in dealerships at the beginning of 2018.
The Autoeuropa plant, opened in 1995 at a cost of 2 billion euros ($2.4 billion at current exchange rates), is one of the biggest foreign investments in Portugal, and one of the nation's biggest exporters.
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