The day-light robbery was caught on a security camera installed in the tunnel.
A delivery agent and his associate were robbed of Rs 2 lakh at gunpoint by four men inside the Pragati Maidan tunnel in New Delhi on Saturday, the police said.
Police said the two men were on their way to Gurugram to deliver a bag full of cash when they were robbed.
No arrests have been made so far.
The day-light robbery was caught on a security camera installed in the tunnel.
The video shows four robbers on two motorcycles waylaying the Ola cab - which the delivery agent had booked from the Red Fort area- as it enters the tunnel on the Ring Road.
The two armed robbers, who were riding pillion, get off the bikes. One of them is seen pointing the gun at the two men in the cab, while the other one collects the cash bag from the backseat.
They then flee the spot.
Police said a case has been registered on the charges of robbery, with attempt to cause grievous hurt.
"It's being investigated if the men were following the duo for a long time. We are also questioning the staff of the company to ascertain if it was an inside job," an official told NDTV.
Delhi Chief Minister Arvind Kejriwal has condemned the daring incident in the heart of the city and hit out at Lieutenant Governor Vinai Kumar Saxena over the law and order situation in the national capital.
"LG shud resign. Make way for someone who can provide safety n security to the people of Delhi. If Central govt is unable to make Delhi safe, hand it over to us. We will show u how to make a city safe for its citizens," Mr Kejriwal tweeted.
Police said about 16 security personnel guard the 1.5 km tunnel, which connects New Delhi with Sarai Kale Khan and Noida. "The security guards were at the entry and exit points of the tunnel when the incident occurred," he said.