(CNN) -- Tournament favorites Spain have suffered one of the biggest upsets in Olympic football history, losing 1-0 to Japan at Glasgow's Hampden Park on Thursday.
Brazil got their Olympics started with a 3-2 win over Egypt in Cardiff, whilst Team GB, playing their first Olympic football tournament since 1960, began with a 1-1 draw with Senegal.
In the afternoon kick off, Spain fell to a goal from Yuki Otsu, who found the net in the 33rd minute, thumping the ball home from a corner. Things got worse for Luis Milla's Spain side as Real Sociedad defender Inigo Martinez was shown a red card minutes before half time.
Spain's Olympic squad has been expected to add Olympic gold to the country's already impressive collection of titles. The Spanish team recently won its second consecutive European Championship, to add to the 2010 World Cup won in South Africa. Spain also hold the European under 19 and under 21 titles.
The European champions enjoyed more of the ball throughout the 90 minutes but they struggled to deal with the pace of Japan's attacks, with David De Gea, Manchester United's young goalkeeper, keeping his country in contention with several fine saves.
Japan, who upset the odds when they beat favorites Brazil at the 1996 Atlanta Games, could have extended their lead as Spain chased a goal, but Nagai and Hotaru Yamaguchi both failed to convert easy chances.
Brazil demonstrated why many feel they are the team to challenge Spain for the gold medal, with an explosive opening 26 minutes that saw the South American side take a 3-0 lead against Egypt. The much lauded Neymar drove a fluid attacking performance.
It was Neymar, who still plays in Brazil despite repeatedly being linked with some of Europe's biggest clubs, that provided a perfect pass for full back Rafael to open the scoring, before in the 16th minute, providing a low cross that invited Leandro Damiao to double the lead.
With 26 minutes gone the 20 year old surged through the Egypt defence before exchanging passes with Hulk, on the left wing, and burying the ball past goalkeeper Ahmed Elshenawi.
The five time World champions struggled to continue their dominance in the second half, with Egypt captain Mohamed Aboutrika pulling a goal back in the 52nd minute.
Mohamed Salah increased the pressure on Brazil, who are seeking to win their first Olympic football gold medal, with his goal 14 minutes from the end, but Egypt couldn't find an equalizer.
In the evening's other Group C game, Belarus enjoyed a 1-0 victory over New Zealand, with Dmitry Baga scoring the winning goal. New Zealand goalkeeper Michael O'Keefe failed to clear a corner on the stroke of half time, and Baga was left with a simple header to give his side the lead.
The reformed Great Britain side, who haven't competed in the Olympic football competition since 1960, made a steady, if uninspiring, Olympic return at Old Trafford, Manchester. Craig Bellamy opened the scoring for the host nation in the 20th minute, but Stuart Pearce's team couldn't find a second goal.
With eight minutes to go, Senegal striker Moussa Konate pulled his side level with a goal against the run of play. Senegal broke quickly after intercepting a poor Great Britain corner, and Konate did well to reach a through ball from Pepe Souare and chip it over the onrushing Britain goalkeeper.
Current South American champions Uruguay got their Olympics off to a good start with a 2-1 win over the United Arab Emirates.
UAE took a surprise lead when captain Ismael Matar took the ball around Uruguay goalkeeper Matin Campana and slotted it into the bottom corner. Uruguay talisman Luis Suarez dragged his side back into the tie, with a key role in both Gaston Ramirez's equalizer and Nicola Lodeiro's winner.
Uruguay's three points sees them top Group A, with Great Britain and Senegal following with a point a piece.
Earlier in the day, Spain's group rivals Honduras and Morocco got the men's tournament underway with a 2-2 draw. the result ensured Spain are bottom of the group after the first round of matches.
Abdelaziz Barrad opened the scoring for Morrocco, before Honduran forward Jerry Bengtson equalized with a close range tap in. Bengtson, who plays for New England Revolution in the U.S., then put his side ahead with a 65th minute penalty.
The lead only lasted two minutes though, Zakaria Labyad chipping the ball over Honduras goalkeepr Jose Mendoza in the 67th minute. Moments later Morocco's Zakarya Bergdich was sent off for kicking out at Mario Martinez, but the African side managed to hold on for a point.
Mexico and South Korea got Group B underway with a goalless draw at St. James' Park, Newcastle. Substitute Raul Jimenez struck the post for Mexico in injury time, while South Korea captain Jacheol Koo went close with a glancing header.
It was a disappointing game between two sides with genuine hopes of making it to the knockout stages.
In the other Group B game, Gabon and Switzerland tied 1-1 after Pierre Aubameyang saved a point for the African side after Admir Mehmedi gave the Swiss side the lead with a fifth minute penalty.