An ex-army officer who allegedly bludgeoned six people to death with an iron-rod near New Delhi on Tuesday has been arrested by the police, officials said.

The killing rampage, which occurred over the course of 90 minutes, sent shockwaves across Haryana state's Palwal city, prompting the police to declare a high alert and ask people to stay indoors.

City police said the murders took place in different areas of Palwal and the suspect, identified as Naresh Dhankad, 45, could be mentally unstable.

Dhankad first murdered a woman attending to her relative at a hospital and later attacked other victims, mostly security guards, police officer Mukesh Kumar said.

"We tracked him down after the CCTV footage showed a man holding a rod and looking around. He was nabbed by a police team around dawn when he was chasing another man, attempting to kill him," Kumar said.

The man, formerly a lieutenant in the army, had taken voluntary retirement and was working in the state agriculture department.

"We have launched investigations to ascertain the cause behind the massacre. He did not know the victims or attempt to rob them so we can only surmise he is likely suffering from some psychological problem," the officer said.

The suspect was taken to a government hospital and was being treated and examined by doctors, police added.

He was facing trial for assaulting a policeman in a previous criminal case.