The largest fire was raging in the central district of Castelo Branco where temperatures have topped 40 degrees centigrade (104 degrees fahrenheit) in recent days.
"This is the blaze which worries us most. We are sending reinforcements from other districts to help fight it," said the duty officer at the National Rescue Operations Centre, Antonio Gualdino.
All but one of the fires was burning in central and southern Portugal where temperatures have been at their highest.
The firefighers were being aided by 85 vehicles and seven water-dropping helicopters, but high temperatures combined with low air humidity and strong winds were making it difficult to put out the flames, Gualdino said.
Portugal's department of health issued a heat warning for five central and southern districts earlier on Wednesday under a new alert system introduced last month.
The alert level was ratcheted up there from "blue", or normal, to "yellow", which cautions that high temperatures could have adverse affects on health. There are two other higher levels in the alert system.
By raising the alert to "yellow" hospitals are required to boost their capacity to deal with patients and they must increase their monitoring of possible affects of the heat on the population.
The system was devised to counter the effects of extreme summer temperatures which were responsible last year for the deaths of about 2,000 people.
The heat wave in 2003 was the longest and hottest in Portugal since records began in 1856 and was part of a wider spell of extreme temperatures that gripped much of Europe.
The dry heat also sparked a rash of forest fires which caused over one billion euros (1.2 billion dollars) in damage and claimed 20 lives, including those of two firefighters.