The 93-year-old former PM's condition is stable, the hospital said in a health bulletin late last night, adding that many tests were being carried out. This morning, a team of doctors assessed his health and it will decide on whether and when to discharge him.
Mr Vajpayee's treatment is being monitored by Randeep Guleria, the director of AIIMS, who was Mr Vajpayee's personal physician for over three decades.
Prime Minister Narendra Modi, Congress president Rahul Gandhi and other senior politicians visited the BJP veteran, who was admitted in the hospital around 11.30 am on Monday.
After his visit, PM Modi's office tweeted:
"PM @narendramodi visited former Prime Minister Shri Atal Bihari Vajpayee at AIIMS today. He was at AIIMS for about 50 minutes. The Prime Minister interacted with family members of Shri Vajpayee. He also spoke to doctors and enquired about the health of Shri Vajpayee. pic.twitter.com/CctZYDJV8o"
- PMO India (@PMOIndia) June 11, 2018
Mr Vajpayee, the first non-Congress PM to complete a five year term, has been away from public life since 2009, and has not been seen leaving his home in Delhi.
In 2009, he was admitted to the hospital just 48 hours before a party's leadership meeting in Nagpur. That was when he stepped away from active politics.
Mr Vajpayee is regarded as one of the tallest leaders of his party. In 2015, the former Prime Minister was honoured with the highest civilian award, the Bharat Ratna.