Cardiff Blues took on the Ospreys at 14:45 BST followed by Newport Gwent Dragons against the Scarlets.
The transport network was expected to be busy and there was a full city centre road closure from 12:45 until 20:15.
There were also more police officers than usual on the streets.
Cardiff council said it was not "a reflection of any specific threat but the current global climate".
Arriva Trains Wales put on 2,000 additional seats for last year's tournament, but fans reported some issues with crowding on trains.
Those travelling by train this year were advised to arrive in Cardiff three hours before kick-off. Queen Street Station was closed at 16:30.
All those leaving the stadium after the rugby have been advised to return to Cardiff Central Station, where a queuing system has been put in place, straight after the event.
Those travelling into the city by car were advised to use the park and ride facilities at Cardiff City Stadium.
Event day parking was also available at Sophia Gardens and in the Civic Centre.
A full list of road closures can be found here.
People heading to Cardiff on Saturday were urged to plan ahead as more than 60,000 were expected to watch the rugby at the Principality Stadium.