Mozambican doctors launched a strike on Monday over pay and working conditions after talks broke down with the government, the country's doctors association said.
Union leaders said the initial action was for five days, but it would continue beyond that if they had reached no agreement.
The health ministry however warned that the law did not allow such an action and that striking doctors would have their pay docked.
"We're on strike for five extendable days until the government meets our demands," said association head Jorge Arroz.
"The strike is the ultimate tool by which the doctors can be better served, though we ensure that emergency services are functioning," he told AFP.
There would be no street demonstrations and the strikers would stay at home, he added.
Although military doctors stepped in to try fill in the gap, patients at the country's largest hospital, the Maputo Central Hospital, were forced to queue for hours.
"Since I arrived this morning, at 7 o'clock, I could see that the consultations were very slow,... but I am sick so I had to come", said Tenday Nhica, 46, in a waiting room crammed with mostly women and children.
In smaller medical centres, health technicians and nurses were providing basic services and care.
The health ministry has already warned that the law in Mozambique does not allow essential public service workers to strike. Officials have also said that the stoppage is illegal because the association cannot act as a union: the country does not have a medical workers' union.
Ministry spokesman Mouzinho Saide said Monday that striking doctors would have their wages cut for every day they failed to work.
Maputo Central Hospital chief Domingos Diogo said of the doctors claims: "Nobody said they were not right to ask for better conditions."
But the strike action was not justified, he said.
Last month, local media reported that doctors were demanding a basic wage of 25,709.4 rands while the government was offering between 20,000 and 38,000 meticals.
Mozambique has a total of 1,200 doctors in both public and private practice countrywide, with a ratio of one doctor to 22,000 Mozambicans.