(Adds news of arrest, police chief quotes, details of attack)
MUMBAI, Aug 23 (Reuters) - A photo journalist was gang-raped in the Indian city of Mumbai, police said on Friday, evoking comparisons with a similar assault in New Delhi in December that led to nationwide protests and a revision of the country's rape laws.
The attack on Thursday night triggered protests and an outcry on social media, with many users shocked that it took place in Mumbai, widely considered to be India's safest city for women.
One man was arrested on Friday and 20 police teams were pursuing four men who had been identified, said Mumbai Police Commissioner Satyapal Singh.
"Mumbai police will do its best to collect all the evidence - clinching evidence, scientific evidence - so that a fool-proof case is made out in the court, and they get maximum punishment," Singh said. "We will also request the government that this case be conducted in a fast-track court."
In rowdy scenes in the upper house of parliament, opposition lawmakers accused the government of not doing enough to protect women, despite tougher sex crime laws brought in this year.
The victim, who is in her early twenties, was admitted to hospital in south Mumbai where she was in stable condition, a hospital official told Reuters by e-mail.
The attack took place shortly before sunset in an abandoned textile mill in Lower Parel, a gritty former industrial district that is now one of the city's fastest-growing neighbourhoods of luxury apartments, malls and bars.
The woman was at the mill on an assignment with a male colleague. The pair were separated by the attackers and her colleague was tied up with a belt while she was assaulted, Singh said.
Several dozen mainly male supporters of the right-wing Shiv Sena political party gathered with flags and banners outside the police station where the case was filed. A further protest was called for later in the afternoon.
Women's safety in India has been in the spotlight this year following the brutal gang-rape of a 23-year-old student on a moving bus in New Delhi in December, which led thousands of Indians to take to the streets in protest. The woman died of her injuries two weeks later in a Singapore hospital.
The trials of the four men and one juvenile accused of the December attack are expected to conclude within the next three weeks. The verdict on the juvenile suspect is set for Aug 31. Closing arguments in the trial of the four adult suspects started on Thursday.
Following public outcry over the Delhi attack, India introduced tougher rape laws in March, which include the death penalty for repeat offenders and for those whose victims were left in a "vegetative state".
In contrast to Delhi, Mumbai has long been considered a safer place for women to travel alone, even at night.
"(Mumbai) has this sense of security ... but these things make us feel that maybe we are not really that safe," said A. L. Sharada, director of Population First, a Mumbai-based NGO that works on women's rights issues.
"Women should be able to move freely and take up work. Why should we be worrying about something bad happening to us all the time?" (Reporting Aradhana Aravindan in MUMBAI and Shyamantha Asokan and Aditya Kalra in NEW DELHI; Writing by Shyamantha Asokan; Editing by John Chalmers)