The network's 400,000 daily users were forced onto buses to get to work, with all stations in the capital closed until early Saturday morning.

Union leader Anabela Carvalheira claimed that the Spanish transport group Avanza, which last week won the tender to run the city's bus and metro system, would "sack workers and rehire on lower wages".

Portugal's centre right government has already raked in nine billion euros ($10 billion) from selling off state assets to pay back its creditors, who bailed out the debt-ridden country in 2011.

But with a general election in four months, the sell-off of the metro has been hotly contested by Lisbon's socialist mayor, with unions claiming it could also lead to a hike in fares.