Celebrations began on 6 November with Dhanteras, when buying gold, silver or new utensils is considered auspicious and believed to bring good fortune.
The third day is the main celebration. Families offer evening prayers to Lakshmi, the Hindu goddess of wealth and prosperity, then gather for festive meals and fireworks.
Although customs differ by region and community, Diwali is widely marked by prayers, family gatherings, the exchange of sweets and the lighting of firecrackers.
The festival falls on the darkest night of the Hindu lunar month of Kartik, when the moon is not visible.
Colourful patterns known as rangoli are drawn at entrances, while markets fill with shoppers buying festive goods under bright decorations.
The festival is also treated as an auspicious time for new purchases, which many believe bring prosperity and good fortune.
Homes, temples, offices and streets are being illuminated with oil lamps and colourful lights, according to the BBC.
The fourth day is observed differently across India, with family gatherings and New Year festivities in some communities.
In the days before the festival, families clean and decorate their homes with flowers, colourful lights and traditional oil lamps known as diyas.
Millions of Indians are marking Diwali, the festival of lights, with glowing lamps, fireworks and traditional sweets.