Cardiff has ground to a halt for the biggest Six Nations match the Wales team has played since 1978.
Thousands of supporters will watch Wales - in and out of the stadium
A crowd of tens of thousands of Welsh and Irish fans packed pubs, bars and the lawn in front of a big screen at City Hall to see their teams clash.
The Millennium Stadium was also a full house with almost 73,000 fans inside.
The Welsh crowds are hoping to witness their side win their first Six Nations Grand Slam for 27 years.
And the omens were good after Wales' under-21s and the Welsh Fire Service team both won Grand Slams on Friday night by beating Ireland.
Emergency services and pubs are ready for a night busier than New Year's Eve.
Revellers reported having to queue for entry to some bars as early as 1100 GMT, as people picked out their vantage points in front of television screens for the biggest match at the Millennium Stadium since it was opened for the rugby world cup in 1999.
Another large crowd gathered to watch a big screen set up outside City Hall before the 1530 GMT kick off.
Singers Max Boyce, Katherine Jenkins and Charlotte Church led the singing of the Welsh anthem inside the stadium after the teams ran out onto the pitch.
And also before the match began, three Royal Welch Fusiliers who were awarded Military Crosses on Friday for bravery whilst serving in Iraq were presented to the crowd.
A queue forming outside a pub in Cardiff's morning sunshine
The men - Sergeant Major Mark Evans, from Rhyl, Fusilier David Evans, from Swansea, and Sergeant Neil Griffiths, from Pontypridd - all took up invitations from the Welsh Rugby Union to celebrate their honours.
On Friday, Wales had early reasons to cheer as two of its teams beat Ireland in their own Six Nations contests.
Wales' under-21s overcame Ireland's by 32 points to five, giving the team the Grand Slam title and boding well for the future of Welsh rugby.
And the Welsh Fire Service team thrashed the Irish Fire Service team by 70 points to 12 at the Glamorgan Wanderers Memorial Ground in Cardiff.
An extra team of rapid response paramedics has been drafted in to deal with the huge numbers of supporters - both Welsh and Irish - due in the city centre.
The crews will be on duty to deal with 999 calls, alongside a mobile casualty unit set up to deal with people with minor injuries.
A spokesman for the Cardiff Community Safety Partnership (CCSP) said: "We are expecting it to be a very busy weekend, as busy if not busier than New Years Eve."
"But because of the nature of rugby crowds traditionally, we are expecting the main problems to be crowd control really," he added.