The attack took place outside the Jama Masjid mosque area in the ancient walled city that is one of the main attractions on tourist routes through the Indian capital.
"Two persons were injured in the firing," New Delhi assistant police commissioner Jagbir Singh told AFP, adding that the victims had been transferred to the nearby Lok Nayak Jay Prakash Hospital.
A doctor there, Vijay Kumar, said the victims, both male, appeared to be of east Asian, possibly Taiwanese, origin and one had a serious stomach injury. "One has been shot in the stomach and is serious, but his pulse and blood pressure appear to be normal," Kumar said.
Eyewitness Hafeez Javed told local Zee television that the attackers fired with automatic weapons. The shooting comes as New Delhi gears up to host the Commonwealth Games, which start in two weeks, with thousands of extra security forces on duty to ensure the safety of athletes and officials.
Indian officials have promised that the event, dogged by delays and overspending, would be secure amid persistent worries over safety from participating nations, notably Britain and Australia.
Chief minister of New Delhi Sheila Dikshit confirmed the firing and appealed for calm. "It's a very sad incident," she told NDTV news channel. "I appeal to everybody: please do not panic. An incident like this is worrying but nothing to panic about."
Police spokesman Rajan Bhagat said that the remaining tourists on the bus were safe and "we have launched a manhunt for the attackers and intensified security".