Roads were closed in and around the luxury Taj Mahal Palace hotel - the focus of the deadly siege two years ago that killed 166 - while armed police were out in force at high-profile sites, including overseas consulates.

Mumbai police warned that four operatives of the Pakistan-based Lashkar-e-Taiba (LeT) militant outfit were preparing a “violent attack” during the festive season and called for residents to be vigilant.

“The four recently sneaked into the city to carry out extremely dangerous activity,” Joint Police Commissioner Himanshu Roy told a news conference on Thursday. “It is going to be a violent attack.”

He identified the four men as Abdul Karim Musa, Noor Abu Elahi, Walid Jinnah and Mahfooz Alam but said their nationalities were not known. All the suspected terrorists were between 20 and 30 years of age.

On Friday, police set up checkpoints along major roads in the city, put additional men on patrol duty at public places and released computer photographs of the four suspects.

However, traffic was normal, and people went about their normal activities in the rest of the city.

Police have received credible information that at least four men belonging to the LeT have entered the city and were planning to strike during the holiday season, said Roy.

“The four have recently arrived in Mumbai. We believe the threat is serious,” Roy said.

“If they have issued an alert of this level it will be credible,” homeland security specialist Ajai Sahni, the executive director of the Institute of Conflict Management in New Delhi, said. “Alerts are issued fairly frequently. They might not have names attached to them as clearly. There seems to be more specific information than there usually is.”

India blames Lashkar for the 2008 assaults. Pakistani officials could not immediately be reached for comment.

It is the second time this year that city police have issued such a warning. In September, two militants were said to be preparing to strike as millions of Hindus thronged the streets of India's financial capital to immerse idols of the popular elephant-headed deity, Ganesha, in the Arabian Sea.

One was said to be a Bangladeshi national and the other from Pakistan. The pair — said to belong to the Harkat-ul-Jihad-al-Islami proscribed as a terrorist organization by the US State Department - have not been traced.

Mumbai police chief Sanjeev Dayal on Tuesday said that intelligence reports indicated there were “conscious efforts by terror organizations to target foreigners during these two festivals here.”

“Extensive security measures” had been put in place and police were “taking things very seriously”, he added.

— Input from agencies