Tens of thousands of well-wishers lined the footpaths as he travelled, followed by marching bands, to a special ceremony at his lavish golden domed palace.
The people waved flags and took photographs during the procession.
It was a very exciting day as Brunei is known for its slow pace of life and lack of nightlife – alcohol is largely banned.
Sultan Hassanal Bolkiah is the world’s second longest reigning monarch behind Queen Elizabeth II.
The absolute monarch rules with an iron fist; there are no elections or free press, and homosexuality is punishable by death.
He is often criticised by human rights campaigners for the country’s strict Islamic laws and lack of democracy.
Thursday’s festivities began with Hassanal, dressed in yellow and gold royal finery and accompanied by his bejewelled wife Anak Hajah Saleha, inspecting a guard of honour in the grounds of his palace, before a gun salute.
The couple then held the royal audience in the throne room of the palace a vast complex of white buildings with golden domes and almost 1,800 rooms.
The event was attended by hundreds of guests including Prince Edward, Earl of Wessex and his wife Sophie, Countess of Wessex.
The prince was dressed in white military gear and medals. He also attended the sultan’s 25th anniversary celebrations in 1992.
The sultan and other members of the royal family travelled through the streets in a convoy of Rolls-Royces before boarding the golden chariot, known as the Usongan Diraja, which has long been used for royal ceremonies in Brunei.
The festivities will continue tomorrow when Southeast Asian leaders and Middle Eastern royalty attend a banquet to mark the jubilee.
Brunei, with a population of about 400,000, is one of the world’s wealthiest nations thanks to abundant oil and gas reserves.
The 71-year-old ascended to the throne in October, 1967.
He is the 29th sultan of Brunei from a royal family that has ruled the country continuously for over 600 years.
The Sultan is believed to be worth US$40 billion. ‘His’ wealth officially belongs to Brunei, but in reality they belong to the royal family – the line between royal family assets and national assets is blurry.
Brunei is the third-largest oil producer in Southeast Asia, and pumps out, on average, 180,000 barrels per day.