Fireworks illuminated the sky in the night in most places as Non-Resident Indians distributed sweets and exchanged greetings to mark the festival that celebrates the triumph of light over darkness.
In 'Ramayana', the ancient Indian text, Lord Rama and his devotees return home after defeating the demon Ravana on Diwali. As the journey took place during the new moon, there was complete darkness. To their surprise however, thousands of local villagers lit up their route by hanging lamps, lighting diyas, candles and fires to help them get home safely.
"I send my best wishes to everyone in Britain and around the world celebrating Diwali and Bandi Chhor (Sikh Diwali). At this important time of year for the Hindu, Sikh, Jain and Buddhist faiths, we will see friends and families all over the country come together to celebrate these holy festivals," Cameron said in a message.
"While the festivals of Diwali and Bandi Chhor have different origins, their universal message of hope over despair, light over darkness and good over evil is something that we should all celebrate.
The Prime Minister also praised Britain's Hindu, Sikh, Jain and Buddhist communities for making an "enormous" contribution to the UK.
"From hardworking families that are doing the right thing to entrepreneurs and businesses that are helping create prosperity; from excelling in science and public service to achieving in the arts and sport, your role in our nation is a shining example of how communities can integrate and succeed while retaining a strong and proud identity," Cameron said.
In one of the biggest celebrations of Diwali in the country, over 15,000 people came together today at Bhaktivedanta Manor Hare Krishna Temple near Watford.
The day's festivities included music, food stalls, drama performances, culminating in a grand firework display and a multi-media presentation.