"There is an alert to intensify security in and around the Bharat Petroleum Corporation Limited and Hindustan Petroleum Corporation Limited refineries following specific information about a possible terror attack," a police officer involved in the exercise told PTI on condition of anonymity.
The security to these installations is provided by the CISF in coordination with state police.
The officer declined to divulge any further details about the terror alert.
The two public sector refineries, located close to Bhabha Atomic Research Centre (BARC), the country's premier multi-disciplinary nuclear research facility in suburban Chembur, produce about 20 million tons of fuel every year.
Security at BARC has also been further tightened, the officer said.
While tracing US national and 26/11 conspirator David Headley's movements in Mumbai, investigators had found that he had conducted a reconnaissance of BARC and some other vital installations.
Headley had travelled to Chembur and Trombay several times and filmed the exit and entry points of BARC, besides the movement of employees.