The racing icon was killed aged 34 at the 1994 San Marino Grand Prix.
During the race, on May 1, 1994, Senna had lost control of his Williams FW16 at the Tamburello corner and hit a wall at full throttle.
Rushed to a hospital in Bologna, he was declared dead just after 6:00 pm.
A triple world champion, Senna was the pride of Brazil and adored by many even outside his nation.
His death shook motorsport to its core and sparked an outpouring of grief around the world.
On Wednesday, the many thousands of fans who flocked to Imola to pay tribute to Senna saw the famous black Lotus JPS which the Brazilian drove in 1985 make some laps of the circuit.
Senna’s McLaren of 1990 and Williams of 1994, the season of his fatal accident, were also on display in the paddock.
A large poster was installed at the track, on which fans could write a tribute.
Messages such as “Best of the best”, “Ciao Legend”, and “Always in our hearts” were left by mourners from Brazil, Italy, Poland, Mexico and Great Britain.
Around the circuit the yellow and green of the Brazilian flags were everywhere, along with t-shirts of Senna and memorabilia for sale.
Just before 2:00 pm, the crowd entered the track and headed for the Tamburello corner where Senna crashed.
Once everyone had gathered, an outdoor mass was celebrated by two priests, one from Imola and the other Maranello, historic seat of the Ferrari team.
It began at 2:17 pm, the exact time of the 1994 accident.
There were also tributes to Austrian driver Roland Ratzenberger, the other victim of the fatal weekend of May 1994, who died the day before Senna, during qualifying.
