Chief Minister Yogi Adityanath, however, expressed doubts over the incident, saying it may have been staged to defame his government, even as opposition parties unleashed a barrage of criticism over the law and order situation in the state.
This is the fifth time the 45-year-old has been allegedly targeted. She had been stabbed in 2012.
The woman was given security after a previous assault this year and the latest attack took place last night near her hostel in Aliganj area of state capital Lucknow, despite the presence of police to protect her.
Less than four months ago, two men allegedly forced acid down her throat on board a train near here -- an incident that had created a huge furore.
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Adityanath had visited her in the hospital and announced assistance of Rs 1 lakh. The police had provided her with security after the alleged attack.
Today, talking to a private news channel, the chief minister said the hostel the woman resides in is absolutely safe and no outsider could get in.
"Ye ghatna sachmuch ghatna hai ya fir ghatna ke naam par kuchh log kewal badnaam karne ki sajish rach rahe hain (Has this attack really happened or are some people hatching a conspiracy to defame the state government)," he said.
He also said the law is meant to protect but if someone misuses it, action would be taken against them.
Police said no FIR has been registered yet and they are awaiting a complaint from the victim's family.
"We are waiting for a complaint to be lodged by the family members of the woman. Police will act on the complaint lodged by them. The husband of the woman could not meet her last night," ADG Lucknow Abhay Kumar told PTI
Terming it an unfortunate incident, she said the police are looking into the matter and have also detained those involved in the previous attack.
The series of attacks on the woman began in 2009 when two men had allegedly raped and attacked her with acid over a property dispute in her home at Unchahar, 100 kilometres from Lucknow.
After the knife attack in 2012, she had again been allegedly attacked with acid in 2013, police officials said.
While she was in the King George's Medical University hospital receiving treatment, three women constables were suspended for allegedly clicking selfies with her.
The police, meanwhile, claimed that none of the eyewitnesses had seen anyone throwing acid or running away.
"Be it the hostel warden or the girls, no one saw the attacker or anyone fleeing from there. We are trying to take the statement of the victim," a police officer said.
"When I heard a voice, I ran towards the hostel, but I did not find anyone running from there. I was there outside," constable Sandeep said.
The Samajwadi Party attacked the state's BJP government over the law and order situation. "They (the BJP) used to make a lot of hue and cry over law and order in the previous SP dispensation. Now, they have lost all control and criminal elements are ruling the roost in the state," SP leader Naresh Agarwal said.
"Attacks on women have increased under the present government. The Yogi Adityanath government has failed to check them," he said.