Around 50 firefighters are battling a blaze on Ministry of Defence land at Middlewick Ranges near Colchester.
Crews have asked people to avoid the area so fire engines can park by hydrants.
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Essex County Fire and Rescue said the fire is on Mersea Road, a site also used by local residents.
The area is home to nightingales and other wildlife.
Residents on Alert
Mike Lilley, a Labour city councillor, said residents were preparing to protect their homes as smoke rose from the range.
“Residents are getting hoses and buckets of sand ready to protect our homes,” Lilley said.
Local resident Aimme Keteca described the smoke as substantial, saying she could see black smoke intensifying from the ground.
“It looks pretty serious, the black smoke is substantial,” said Keteca, 41.
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Witnesses saw the scale of the fire from miles away.
John Hood, watching from Colchester United's football stadium about 3.7 miles (6km) away, said smoke was rising hundreds of feet into the air.
Brian Gilbert said the smoke could be seen from Abberton Reservoir and described it as very large as crews worked on the blaze.
“It looks very, very big,” said Gilbert, 68.
In a separate update on regional wildfire impacts, the ongoing situation in Suffolk has involved multiple fire brigades for a third day.
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Authorities have declared a major incident in parts of the coastal area while habitats at Dunwich Heath are damaged by flames.
