Police Commissioner Rakesh Maria said the letter said he had stopped them in 1993 but wouldn't be able to do so this time.

Mumbai has been put on high alert after the police received a letter purportedly threatening a terror attack in the city to "take revenge for Gaza" attacks.

The one-page letter, addressed to Mumbai Police Commissioner Rakesh Maria, was received by the Control Room on July 25 (Friday) through post.

"We will take revenge for Gaza. In 1993, you (Maria) got a chance. This time you won't. Stop us if you can," Maria said, quoting the letter which he says is signed as "Mujahideen".

The other contents of the letter, written in Hindi and English, hinted at a terror attack in the city, Maria said.

"We are verifying the authenticity of the letter. However, we are not taking the threat lightly. The Crime Branch and all the police stations have been alerted," he added.

Maria had recently issued an internal circular to the police department which has reference to terror attacks at various places in Mumbai as well as the whole state during the "festival season" and asked the personnel to be alert.

Maria, presently accorded Z-category security, had investigated the 1993 Mumbai serial blasts as Deputy Commissioner of Police (DCP), rounding up many members of the Dawood Ibrahim gang and other members of his family.

Since last year, the National Investigation Agency (NIA), Delhi Police and Maharashtra Police have arrested some key Indian Mujahideen operatives, including Yasin Bhatkal.

PTI