The accused had been stalking the victim for four or five days and molested her when she was returning home after celebrating New Year, Bengaluru Police Commissioner Praveen Sood told reporters in Bengaluru, amid growing public anger over delay in nabbing the culprits.
Sood said two other accomplices had been identified and they would be arrested soon.
"This is a plan prepared by six of them. One of them groped. Others assisted," the police commissioner said.
He gave the names of the arrested as Aiyappa, the main culprit who molested the woman and Leno, Sudesh and Somashekar.
Aiyappa and Sudesh are delivery boys, Leno was a helper and Somashekhar a driver, Sood said, adding, a bike had been seized from them. "They have confessed (to the crime)," he said.
The arrest comes three days after a CCTV footage surfaced showing two scooter-borne two men accost a woman, zoom past her, take a U-turn and halt the vehicle before one of them jumps off, gropes her and escapes even as bystanders watch without coming to her help.
Sood said there was no "friendship or anything except that they stayed close by (to the victim's place)."
He said Prashant Francis, a resident of Kammanahalli, came forward with the "gory" video to help police.
Sood said "when we saw the video I felt that I don't have to talk to the victim at all. It was a clear case of molestation and groping and maybe much more. We did not wait, we did not waste time to look for the victim or contact her."
"We suo motu registered the case because what had been shown on the video convinced us beyond all possible doubt that this was a heinous crime."
Sood warned all such "potential violators" that police would not have any mercy on them if such incidents are repeated in future.
In the footage, one of the two men is first seen approaching the woman, forcing himself on her and dragging her to the scooter while the other person is seated on it, before pinning her down.
As she resists and tries to free herself, he pushes her down on the road and both escape from the scene, but not before the victim slaps the one who had grabbed her.
The incident at Kammanahalli here had occurred even as on late December 31 night, several women were molested and groped and lewd remarks passed by miscreants on M G Road and Brigade Road, despite the presence of 1,500 police personnel to control the unruly New Year revellers.
The incident drew flak from many quarters even as Home Minister G Parameshwara stoked a controversy by blaming the youngsters' "western ways" for the incidents.
Bengaluru molestation: Police finds credible evidence, registers FIR
BOO! When women come out in small dresses such incidents do occur, says Azmi
The bizarre reasons why men rape in India
Why no law against rape will work
India's SHAME: 92 women raped every day