Portugal is rationing gas as a precaution after some 2,000 tanker truck drivers began an open-ended strike over pay on Monday.
The government has set a limit of 25 liters (6.6 gallons) for customers at gas stations until further notice.
Some truckers in other sectors of the economy also joined the walkout.
Authorities hope to minimize disruption by demanding that truckers maintain gas supplies to emergency services and airports, provide 75% of normal gas deliveries for public transport systems and 50% to gas stations.
All perishable goods and at least half of normal food supplies must also be delivered to stores.
Some trucks delivering gas Monday had a police escort, and police said they were on standby for the possibility of roadblocks erected by strikers. Few queues were reported at gas stations after many people filled their tanks over the weekend.
The government warned that if truckers do not comply with the stipulated level of minimum services, it will enact a rarely-used law to force them back to work.
At the same time, it is preparing to use qualified members of the security forces to deliver gas, if necessary.
Truckers are demanding staggered annual rises over the next three years in their basic pay and monthly allowances. They want their basic pay to increase from 630 euros ($705) a month to 900 euros by 2022. Their current allowances are indexed to basic pay.
Employers say they can't afford the demands by the unionized truckers.
Prime Minister Antonio Costa, who early Monday visited emergency services to see their operational readiness for the strike, appealed to both sides to return to negotiations.
Farmers fear they won't be able to get fresh summer produce, such as lettuce and tomatoes, to stores and markets before they go rotten. Dairy farmers have warned they could lose millions of liters of milk a day.
Portugal's southern Algarve region, a popular European vacation destination, receives around 1 million tourists in August, and tour operators are concerned holidays could be disrupted.