Police in Mumbai are searching for members of the Lashkar-e-Taiba group believed to be in the city.
Police commissioner Himanshu Roy told a news conference on Thursday evening that there was credible information that at least four members of LeT were in the city.
A sketch of one of the suspects was issued. The BBC reports checkpoints have been set up and extra officers are on patrol.
India blames LeT for the 2008 attacks in Mumbai which killed 166 people.
Roads were closed on Friday in and around Taj Mahal Palace hotel - the focus of the siege two years ago.
Joint police commissioner Himanshu Roy told a news conference on Thursday evening:
"The four men are planning violent attacks that are going to cause destruction.
"The four have recently arrived in Mumbai. We believe the threat is serious," he added.
Police issued a similar warning in September about a possible attack by two Islamists in the city, which is India's financial capital. But the BBC reports nothing happened and no arrests were made.