The two labour groups representing close to a million Portuguese workers say it could be the country's biggest walkout in more than 10 years as they contest the centre-right government's planned changes to employment laws.
The unions say the changes strip workers of entitlements, while the government argues they are needed to make the economy more supple and spur growth.
The proposed changes include making it easier for companies to fire workers, denying the right to strike in additional sectors of the economy and limiting breastfeeding breaks for mothers to the first two years of a baby's life from the current open-ended dispensation.
Lisbon Metro said services were suspended at 11pm on Wednesday and would resume early on Friday morning.
Portuguese Railways warned of disruptions, while the light rail operator in Porto, Portugal's second-largest city, also announced reduced services Thursday due to the strike.
Portugal has one of the European Union's smallest economies and its workers are among the lowest paid in the 27-nation bloc.
The Portuguese are also being pinched by a housing and cost-of-living crisis, as property prices soar and inflation sticks at just over two per cent.
The European Commission expects Portugal to achieve GDP growth of about two per cent in 2025, above the EU average of 1.4 per cent.
Unemployment stands at under six per cent, roughly the EU average.
Trade unions plan street marches later on Thursday.
It is the first time since 2013 that the umbrella groups - the General Workers Union and the General Confederation of Portuguese Workers - have joined forces.
Social Democrat Prime Minister Luis Montenegro described the strike as "senseless" because the country was doing well.
