The fire broke out at the Tilbury 'A' power station around 3 p.m., and eight fire brigades are in attendance.
The fire is thought to have broken out in one of the plant's turbines, but there is no indication if this will impact power generation.
Speaking to the press, a firefighter said, "Firefighters are tackling the blaze, which is in a power station measuring 200 metres by 300 metres 25% of the station is well alight. Eight pumps and an aerial ladder platform are now involved in firefighting operations."
According to RWE npower, the plant was offline for routine maintenance when the blaze broke out.
The Tilbury 'A' station was commissioned in 1956 by the CEGB and was followed by Tilbury 'B' 1968. Today it is operated by RWE npower, part of German power company RWE AG. The station has four generating units with a combined capacity of 1,428-megawatts, enough power for 1.4 million people, approximately 80% of the population of Essex. The station is connected to the National Grid by a nearby 275-kilovolt substation.