The Portuguese government's plan to privatize debt-heavy public transport companies is meeting tough opposition from trade unions as Lisbon subway workers walked off the job for the 12th time this year.

Staff staged a 24-hour strike Tuesday, demanding the government keep the Metropolitano de Lisboa in public hands.

Transport workers are among Portugal's most militant labor groups, and the third stoppage in two months stranded many Lisbon commuters.

The country's public transport companies are around 16 billion euros ($20 billion) in debt and can't invest in improving services and infrastructure.

The government also intends to sell flag carrier TAP Air Portugal and grants concessions to run the Lisbon bus company Carris and parts of national rail company Comboios de Portugal.