"We have declared an alert in Mumbai (following the blast in a local train). Security has been tightened," Police Commissioner Ranjit Singh Sharma told rediff.com on Thursday night.
Visiting injured passengers in a hospital, he sounded concerned but composed.
"Compared to the Ghatkopar blast, today's was much bigger in magnitude. I would not like to say anything more at this stage."
He refused to comment on the nature of the explosive or the modus operandi of the culprits.
"At this stage it's difficult to say if there is any connection with the Ghatkopar blast. I request the public to come forward if they have any clue or information which can lead us to culprits."
The police would be extra careful, as Muslims would be observing the last day of Moharram on Friday.
Kirit Somaiya, BJP MP from Northeast Mumbai, demanded 'strong action from the state government against terrorism in Maharashtra'.
"We have seen four blasts in the last three months. First outside Ghatkopar station, then Mumbai Central station, near Vile-Parle station and now Mulund station while one was averted in Andheri."
"All near railway stations and in areas counted amongst the more peaceful localities in Mumbai. On Thursday, for the first time, the terrorists targeted ladies. This is disturbing," Somaiya told rediff.com.
He criticised the state government for being unable to prevent the blast, which 'occurred a day after the 10th anniversary of serial blasts that had rocked Mumbai'.
"There is a link behind these blasts. Why are they happening at a regular interval? Does it appear like the four blasts occurred for four different reasons?"
"It's high time the government took stern action. It's a shame that terrorist suspect Mohammed Afroz had to be released while an accused in the Ghatkopar blast escaped from police custody. Such incidents erodes the credibility of the government and encourages terrorists."
"I'll ask Centre to extend all possible cooperation to the state government to fight terrorism," he said.