The blaze, fanned by strong winds, affected at least six communities in the foothills of the Troodos mountain range, an area of pine forest and densely vegetated shrubland.
The EU's executive, the European Commission, said fire-fighting planes had departed from Greece to battle the fire and Italy was also planning to deploy aerial fire-fighters.
The EU's emergency Copernicus satellite was also activated to provide damage assessment maps of the affected areas, the Commission said in a statement.
"It is the worst forest fire in the history of Cyprus," Forestries Department Director Charalambos Alexandrou told Cyprus's Omega TV.
Attempts were being made to prevent the blaze from crossing the mountains and stop it before reaching Machairas, a pine forestland and one of the highest peaks in Cyprus.
Alexandrou said the perimeter of the fire was "at least 40 kilometres".
Dozens of properties were damaged, but no injuries were reported. There were widespread power cuts in the area. Plumes of smoke were visible in the capital Nicosia, some 75 km (45 miles) away.
Officials said that in addition to Greece's assistance with two aircraft, help was also expected from Israel.
"This is a very difficult day for Cyprus. All of the state's mechanisms are in gear, and the priority is for no loss of life," Cypriot President Nicos Anastasiades tweeted.
Israel accepted Nicosia's plea for help, a statement from Prime Minister Naftali Bennett said, and will send firefighting aircraft to Cyprus on Sunday.
The cause of the fire, which started around midday, was unclear. Cyprus has experienced a heatwave this week, with temperatures exceeding 40 Celsius (104 Fahrenheit). Police said they were questioning a 67 year old person in connection with the blaze.
"It passed through like a whirlwind, it destroyed everything," said Vassos Vassiliou, the community leader of Arakapas, one of the communities affected. Reporting by Michele Kambas, additional reporting by Maayan Lubell in Jerusalem and by John Chalmers in Brussels Editing by Ros Russell, David Gregorio and Diane Craft