The parade, which lasted for about an hour, finished with a welcome from the Minister for the Welsh language, Eluned Morgan AM.
Cardiff Castle reached capacity by mid-afternoon and there were queues to get into the festival.
The Mid & West AM described the turnout as “fantastic” and said the event was “packed”.
More than 40,000 people attended the event in 2018, compared to around 1,000 in its early days at Y Mochyn Du pub.
The festival was established by the charity Menter Caerdydd in 2006 to celebrate the use of the Welsh language in Cardiff.
It involves music, literature, drama, comedy, art, sports, food and drink. A week-long fringe ends in the main event at Cardiff Castle.