Rugby fans were advised to arrive in good time to watch Wales take on Scotland at the Principality Stadium on Saturday.
Mark Williams, who manages the Cardiff stadium, said the expected 62,000 crowd could not all be processed through the turnstiles in the hour before kick-off.
Enhanced security measures at the stadium meant the gates opened three hours before the 14:45 GMT kick-off.
An alcohol-free zone was in place for the first time.
Cardiff is expected to be busy with sports supporters on Saturday as another 30,000 football fans were also due in the Welsh capital to watch Cardiff City play Leicester in the Premier League at the nearby Cardiff City Stadium at 15:00.
Big screens at the Principality Stadium showed an "as live" version of Japan's clash with New Zealand in Tokyo to entertain fans arriving from 11:45.
"The earlier you arrive, the quicker you will get in," Mr Williams said.
"We will not compromise on safety and our enhanced security measures are here to stay," he said.
"This means that everyone who comes through our turnstiles is searched and the process can create queues which increase as kick-off approaches.
"Queues outside will not delay kick-off and search procedures will be maintained, so the best way to ensure you are in your seats in time is to get in early."
Fans were strongly advised to leave large bags at home and read the prohibited items listed on the stadium's website before travelling into the city.
Cardiff city centre road closures began at 11:15 GMT
City centre road closures will be in place between 11:15 and 17:45. The M4 corridor and approaches to Cardiff are also expected to be very busy.
Transport for Wales advised fans journeying by train to travel as early as possible as they will be busy all day.
There will be a post-match queue system in place at both Cardiff Central and Cardiff Queen Street.
Buses displaced from stops within the road closure area will relocate to either Churchill Way for the east or Greyfriars Road for the north.
Meanwhile, Welsh Rugby Union chief executive Martyn Phillips said he expected more than 2,500 supporters to take advantage of the 4,200 "dry seats" on the second tier of the North Stand.
The alcohol-free zone was introduced for Wales' November internationals on a trial basis after customer feedback during the 2017/18 season suggested it would appeal to some supporters.
The trial will be used to assess the extent of the demand for a zone of this kind, as well as its practical application.