The vast nation of 1.3 billion people recorded 349,691 fresh cases and 2,767 deaths -- the highest since the start of the pandemic.
The northern megacity -- home to 20 million people and the worst-hit in India -- had imposed a weeklong lockdown on Monday as hospitals beds and oxygen supplies ran out.
"We have decided to extend the lockdown by one week," Delhi Chief Minister Arvind Kejriwal said in a video statement.
"The havoc of corona(virus) continues and there is no respite. Everyone is in favour of extending the lockdown."
The announcement came as the healthcare system struggled to cope with the huge spike in cases, with reports of overwhelmed hospitals, severe oxygen and medicine shortages, and patients' families pleading for help on social media.
There has been growing criticism of the government over allegations that it was caught underprepared ahead of the surge.
The government has stepped up its efforts to provide oxygen supplies through special trains and airlifts of containers from other countries.