India's Prime Minister Narendra Modi has sent his condolences to those who lost loved ones in the blast.
Opposition leader Rahul Gandhi said in a statement that the news was "extremely heartbreaking".
The federal home minister Amit Shah said teams from India's National Security Guard, and National Investigation Agency are investigating the explosion, along with forensic experts.
"We are exploring all possibilities and will conduct a thorough investigation, taking all possibilities into account. All options will be investigated immediately and we will present the results to the public," he added.
The minister, who visited the site of the explosion and a nearby hospital, said a meeting involving senior officials would be held on Tuesday morning.
Security is being stepped up at "sensitive religious sites, vulnerable districts and border areas" in the northern state of Uttar Pradesh, according to senior police official Amitabh Yash following the blast in neighbouring Delhi.
Uttar Pradesh is home to famous sites such as the Taj Mahal and is a densely populated state.
