The first fully underground phase of Delhi's Metro rail network has been launched by president of the ruling Congress party, Sonia Gandhi.
The new line runs under the old city
The 11km (seven-mile) line connects the heart of the capital with the old city.
Delhi's hi-tech metro system was launched in 2002, with the first section using elevated tracks.
The system, which will eventually cover 60km, is seen as the answer to Delhi's traffic problems and it is hoped will lower air pollution levels.
"The face of the city will change when the Metro becomes fully operational. Conditions of roads will improve and traffic jams will be reduced," Mrs Gandhi said.
She paid tribute to the 22bn rupee ($506m) project, which came in eight months ahead of schedule.
Proposed metro system in Delhi
Delhi Chief Minister Sheila Dikshit said: "Not just Delhi, but the whole of India can be truly truly proud of the Metro."
A large part of the line is built under old Delhi, which contains buildings from the 17th century Mughal empire.
Line 2 of the Delhi Metro connects the main government area with the commercial hub of Connaught Place before heading north to old Delhi and Delhi University.
The stretch, which has some of the city's most congested streets, will be covered in 13 minutes.
The system is fully air-conditioned and the stations are equipped with computerised ticketing systems dispensing smart cards.
The next 32km stretch of the Metro - the system's third line - is due to open in December.
There are plans are on to extend the network in time for the 2010 Commonwealth Games being hosted in the city.