Of the 511 arrivals and departures scheduled for Sunday, 301 were cancelled at Lisbon airport and 26 other flights were cancelled at the Porto airport, the Xinhua news agency reported.
"Due to the strike of the handling service," said ANA in a statement, "We appeal to passengers with cancelled flights not to go to Lisbon airport and seek information through other channels, digital and telephone."
Only low-cost airlines that use Terminal 2 at Lisbon airport maintained their regular operation because they are served by other handling companies.
Groundforce's strike began on Saturday with the cancellation of 260 flights at the two main Portuguese airports on the day. Another stoppage was scheduled between July 29 and August 2.
The strike was called by the Union of Airport Handling Technicians (STHA) as a protest against the "unsustainable instability, regarding the timely payment of wages and other pecuniary components" that Groundforce workers have faced since Feburary 2021.
Groundforce is 50.1 per cent owned by the Pasogal Group and 49.9 per cent by the TAP group, which in 2020 became controlled by the Portuguese state.
Groundforce accused TAP of a debt of 12 million euros ($14.17 million) for services already provided to the latter, but TAP said that it has no arrears to Groundforce.