Hosts Britain, making their first appearance at the Games for 52 years, were denied a win in front of a 72,000 crowd at Old Trafford when Senegal scored a late equaliser in a 1-1 draw.
Uruguay, back in the Games after an 84-year absence, came from behind to beat a lively United Arab Emirates side 2-1 in the earlier game at Old Trafford.
On a day when all 16 teams were in action, there was also a win for Belarus, playing in their first major international tournament, beating New Zealand 1-0 at Coventry.
The real upset though was at Hampden Park where Japan stunned Spain, looking to become the first country to hold the World Cup, European crown and Olympic title at the same time.
The Spaniards finished the Group D match with 10 men after midfielder Inigo Martinez was sent off just before half-time having found no answer to Yuki Otsu's 34th minute goal.
Asked if this was Japan's best ever win coach Takashi Sekizuka said: "We looked forward to the match very much and showing how much we have to offer was the main point.
"Now we have to prepare for winning the games against Morocco and Honduras but I am very pleased with the result today."
Despite Spain's hopes of winning the title, Brazil are favourites to take their first Olympic football gold.
They began with an ultimately nervy 3-2 win over Egypt in Cardiff after goals from Rafael, Leandro Damiao and Neymar had put them 3-0 ahead after only 30 minutes.
Egypt rallied in the second half with goals by Mohamed Aboutrika and Mohamed Salah with 14 minutes to play.
Neymar, who scored from a header, told reporters: "We need to improve, but it is always better to improve for the next match after a win."
Britain, playing in the finals for the first time since Rome 1960 were held to a 1-1 draw by debutants Senegal at Old Trafford after leading for more than an hour.
Craig Bellamy put them ahead in the 20th minute but Moussa Konate equalised for Senegal eight minutes from time.
Uruguay, back at the Games for the first time since winning the second of their two golds in 1928, preserved their record of never losing in the Olympics, coming from behind to beat the United Arab Emirates 2-1 in the opening match at Old Trafford.
ALWAYS BOOED
Substitute Nicolas Lodeiro scored Uruguay's winner while captain Luis Suarez was booed every time he touched the ball by a Manchester crowd unforgiving of the Liverpool player's racial spat with United's Patrice Evra last season that earned him a lengthy ban.
Suarez said: "This is just one more match. I'm used to playing in stadiums where I'm booed. It doesn't worry me in the least. They boo me at almost every stadium in England, it's normal. I'd rather highlight the support from the Uruguayan fans."
In Thursday's other matches, Morocco and Honduras
