The man, Ramesh, was allegedly beaten up by the mob near Mangolpuri railway station between 2.30-2.45 am.
According to the woman's complaint, two men came on a scooter and one of them snatched her mobile phone outside her shanty. When she tried to catch them, two of their accomplices came and started beating her.
When she raised alarm one of them took out a knife and tried to stab her. Meanwhile, locals gathered and caught one of them while three others managed to escape, a senior police official said.
The man was later rescued by the police and rushed to a hospital, the police said.
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After treatment, he was arrested and brought to Mangolpuri police station for questioning. He later told police personnel that he wanted to sleep and after he slept, he did not wake up, the police said.
Two separate cases have been registered in connection with the robbery attempt and another with the man's death, they said.
A senior police officer will conduct inquiry into the matter and submit a report in this regard at the earliest, the police added.