The bomb killed four people and left 42 injured as it shattered the rear of the bus in suburban Mumbai during Monday evening peak hour traffic.
"A high alert has been issued in Mumbai as well as other parts of the state," city Joint Police Commissioner Ahmad Javed said.
"We have stepped up security at all the key areas and installations of the city and increased vehicular checking at various entry points of the city and the state," he said.
Investigations were continuing and forensic experts were analysing the explosives used in the attack, the joint commissioner added.
|"A high alert has been issued in Mumbai and other parts of the state"|
--Mumbai Joint Police Commissioner Ahmad Javed
State Home Minister Kripashankar Singh said additional police forces will be called in to boost security in the towns of Nashik and Trimbakeshwar, about 300 km north of
Mumbai, where an ancient Hindu fair begins on Wednesday.
More than five million people are expected to attend the month-long fair, believed to be the largest such gathering anywhere in the world.
The minister said 10,000 policemen will stand guard during the fair in the two towns where Hindu pilgrims take a “holy” bath in the river Godavari.
The "Kumbh Mela" (Fair of the Nectar Pot) is celebrated once every three years in rotation at four different places in the country. This time it is in these two western Indian towns.
|India's top metropolis|
shaken yet again
"If required, additional forces will be called in. We have centre and state reserve forces, home guards and other security units that will be called in depending on the situation," Singh said.
Two state opposition parties, the Hindu Shiv Sena and Bharatiya Janata Party said they had called a one-day general strike on Wednesday to demand better security in the city.
The victims of the bomb attack, meanwhile, are being treated in two local hospitals.
The blast, the fifth in the city in the last eight months, has been blamed by officials on the Islamic separatist group, Lashkar-e-Taiba, fighting against Indian rule in disputed Kashmir state.