"The fire in Delhi's Anaj Mandi on Rani Jhansi Road is extremely horrific. My thoughts are with those who lost their loved ones. Wishing the injured a quick recovery," the prime minister said in a tweet.
"Authorities are providing all possible assistance at the site of the tragedy," he said.
Congress leader Rahul Gandhi also expressed his grief at the accident and said he was extremely sad at the news of the death of many people and injuries to many, due to the huge fire. "I express my deepest condolences to the family of the deceased and wish the injured a speedy recovery," he tweeted.
A massive fire swept through a factory in Anaj Mandi area of north Delhi on Sunday morning, leaving 43 labourers dead and many others injured, police said.
Home Minister Amit Shah termed the incident as tragic and instructed concerned authorities to provide all possible assistance.
"Tragic loss of precious lives in the fire accident in New Delhi. My deepest condolences with families of those who have lost their loved ones. I pray for the early recovery of the injured. Have instructed concerned authorities to provide all possible assistance on urgent basis," he tweeted.
Delhi CM Kejriwal had said: ""Very very tragic news. Rescue operations going on. Firemen doing their best. Injured are being taken to hospitals," CM Kejriwal tweeted about the incident."
Working president of BJP, JP Nadda also tweeted: "Extremely anguished by the loss of lives in fire tragedy at Anaj Mandi, Rani Jhansi Road, Delhi. My thoughts are with bereaved families. May the injured recover quickly. Have asked @BJP4delhi karykartas to provide all possible assistance to those affected."
Congress leader Priyanka Gandhi also conveyed condolences to the families of the deceased who lost their lives in the incident. "The news of the fire in Delhi's Anaj Mandi is very disturbing. I offer my condolences for the families and the loved ones of the deceased. I wish the injured to get well soon. I also hope that the rescue and relief situations are carried out smoothly and swiftly," Gandhi posted on Twitter.
Congress interim Chief Sonia Gandhi has expressed "deep shock" at the massive fire. In a statement released by the Congress, Sonia expressed her "deepest condolences to the families and loved ones of over four dozen deceased in the tragic accident". She said that she sincerely hopes that precious lives will be saved by the authorities and the injured are given due treatment expeditiously. She urged the authorities of BJP-led central and Aam Aadmi Party-led state governments to provide every assistance to victims and their families. She has also instructed Congress leaders and workers to assist the authorities in rescue and relief operations.