Director of the National Rescue Operation Centre, Antonio Gualdino, told reporters the situation was “a bit calmer" after cooler overnight temperatures in recent days.
Higher humidity levels have also helped firefighters extinguish or contain all but four large fires which are currently burning in the centre of the country.
But with winds expected to pick up and temperatures set to rise, Gualdino said firefighters remained on alert for flare-ups.
"Obviously we are very worried about this, with more heat the humidity will drop again and that complicates things," he said.
The weather office is forecasting temperatures will hit 42 degrees Celsius in some regions in the interior of Portugal on Wednesday.
Portugese police have so far detained 26 suspected arsonists, and the head of Portugal's judiciary police, Adelino Salvado, said some 400 officers were taking part in the investigation and more arrests were likely to follow in the coming days.
He said the arrests had been made since the first major fires broke out in central and northern Portugal in mid-July.
Six youths were arrested on Wednesday on suspicion of having set fire to pine forest near the central town of Leiria over the weekend, Salvado said. That fire destroyed 2,500 hectares of land.
Asking for help
Earlier, Prime Minister Jose Manuel Durao Barroso's government appealed to the North Atlantic Treaty Organization (NATO) to provide water-dropping aircraft to help fight the fires.
But a NATO official in Brussels said on Wednesday it would be difficult for the military alliance to meet Lisbon's request.
France, Spain and Italy are also struggling against forest fires as Europe is gripped by heatwave.
Lisbon declared the fires, which have ravaged more than 50,000 hectares of woodland, a national disaster and said it is seeking financial aid from the European Union.
Electrical company EDP said the fires have destroyed some 2,000 kilometres of power lines and services were likely to be disrupted throughout the country over the next few months.
About one-third of Portugal is covered by forest and each year thousands of trees are lost to flames during the hot, dry summer – but this year the flames have not only claimed more than a dozen lives but were probably the result of arson.