Police have called in reinforcements to increase security and guard against any possible crowd problems, although, the early morning mood was friendly and festive.
Authorities estimate more than 100,000 fans have crossed the border between Argentina and Brazil's Rio Grande do Sul state but only about 18,000 of them had tickets, meaning many were hunting for resales.
Hotels have been packed and many have camped by roadsides, slept in cars and camper vans, or just stayed up all night partying. Restaurants and bars were full of singing, chanting fans on Monday night.
With several hours to go until the 1600 GMT kickoff, squadrons of mounted police patrolled the streets and security forces in riot gear took up positions in the avenues leading to the Beira Rio stadium.
Many of the fans wore Argentina's distinctive sky-blue and white striped soccer shirt. One banner showed Pope Frances - an Argentine - alongside star striker Lionel Messi and playing legend Diego Maradona.
"We slept in the car," said Hugo Audalan, a 32-year-old office worker, who had made an 800 km journey from Santiago del Estero with his friend Sebastian Gonzalez. "We have no tickets. We are looking for resales."
"We are here because of what football means to our country. It is our passion," said Gonzalez, 34, who works in public relations.
Audalan reckoned Argentina would win 2-0, with one goal at least from Messi. Gonzalez predicted 3-0.
Another fan, Jose Cibils, had a ticket and a hotel room. He wore a sky blue and white wig, an Argentina shirt, and was wrapped in the national flag. He had arrived by car on Monday from Posada and had a ticket and a hotel room.
Asked about the blanket security, he said: "It's good. This is very well-organised. There's a good mood, very respectful."
The 34-year-old car salesman also had no doubt Argentina would win. But he would not stay on in Porto Alegre to celebrate.
"I have to be back at work tomorrow morning," he said.
One of the favourites going into the tournament, Argentina, champions in 1978 in 1986, have already qualified for the last 16 but Messi aside, they have yet to show their full potential and coach Alejandro Sabella has conceded they must improve.
Argentina lead the group on six points and a draw would see them finish top. Reigning African champions Nigeria need a point to guarantee qualification. Iran and Bosnia and Herzegovina are the other two teams in the group.
- Sports & Recreation
- Lionel Messi
- Sebastian Gonzalez