Brazil got their Olympic campaign off to a winning start as they resisted a second-half comeback to beat Egypt 3-2.
Brazil had looked like the team to beat for 45 minutes, with new Chelsea signing Oscar playing a starring role as they went three goals ahead inside 30 minutes.
Manchester United's Rafael drilled home the opener from, Oscar's assist, and the midfielder was again heavily involved when Brazil doubled their lead ten minutes later as he rounded the advancing Egyptian goalkeeper Ahmed Elshenawi before squaring for Leandro Damiao to fire home from close range.
Neymar made it three a few minutes later as he headed home after combining brilliantly with Hulk - the pair making light of a badly cut-up playing surface.
Egypt pulled one back early in the second period through Mohamed Aboutrika, and then reduced the deficit to a single goal through Mohamed Salah, but were unable to find an equaliser.
One of the favourites for the gold medal, Spain, put in a terrible performance as they had Inigo Martinez sent off and lost 1-0 to Japan.
Spain should have lost by a lot more as their passing game deserted them and they went behind as Yuki Otsu arrived unmarked to meet a corner and give Japan an unlikely lead.
Then less than ten minutes later Martinez was given his marching orders for hauling down his man when through on goal and Spain were unable to get back into the game.
Craig Bellamy's first-half goal for Great Britain was not enough as they started their London 2012 campaign with a 1-1 draw against Senegal at Old Trafford on Thursday.
Having been played off the park by Brazil in a recent friendly, Britain made a confident start against a combative Senegal side, and they were ahead on 20 minutes thanks to a goal made and scored in Wales. Ryan Giggs delivered the teasing cross from the right, and when the ball bounced down inside the area Bellamy mastered a difficult piece of technique to volley into the far corner.
That lead almost lasted a matter of minutes as young 'keeper Jack Butland made a horrible mess of a simple clearance, playing Sadio Mane clean through on goal. Butland spread himself to make life difficult but, having lifted the ball over the keeper, Mane will have been disappointed to see it drift wide.
Team GB came under greater pressure in the second period, with Butland twice forced to make instinct saves to guard his clean sheet, while Danny Rose was also needed to head off the line. At the other end, Bellamy was denied a blatant penalty when he was crunched inside the area 20 minutes from time, and he was withdrawn soon after.
Soon afterwards, Britain conceded, the defence getting the offside trap all wrong as Moussa Konate ran clear to delicately lift the ball over Butland. Marvin Sordell had a glorious chance to win it for Team GB at the death, but his left-footed strike rebounded off the bar.
Uruguay came from behind to beat United Arab Emirates 2-1.
UAE took a shock lead through skipper Ismaeil Matar midway through the first-half and for a time they threatened to extend their advantage, but Uruguay battled back.
They were level on 42 minutes when Luis Suarez - who had been booed throughout by the Old Trafford crowd - had been fouled 20 yards out and Bologna star Gaston Ramirez curled home.
Uruguay looked a new team after the half-time team talk, and 11 minutes after the break they were ahead. Suarez was involved again, cutting inside after collecting a pass from substitute Nicolas Lodeiro, who kept going and buried a clinical shot into the far corner.
Earlier, the men's Olympic tournament started off with a great game as Honduras and Morocco played out a thrilling 2-2 draw at Hampden Park.
Morocco took the lead through Abdelaziz Barrada but a double from Jerry Bengtson put Honduras in command, with the second coming from the penalty spot.
The Central Americans were only ahead for two minutes though as Zakaria Labyad netted an equaliser before Zakara Bergdich was sent off for kicking out as his opponent.
Mexico and South Korea played out a poor 0-0 draw with Raul Jimenez coming the closest when he almost handed the Mexicans a late win when through on goal.
The striker clipped the outside of the post after being put through by Tottenham's Giovani dos Santos with the whistle fast-approaching.
Korea enjoyed the better of the game though with Nam Taehee firing high and wide with 19 minutes gone and Park Jongwoo crashing a long-distance half-volley over seven minutes before the break. Koo Ja-Cheol also headed Kim Changsoo's cross past the post when he might have done better ten minutes from time.
Pierre Aubameyang denied ten-man Switzerland an opening Group B victory as Gabon claimed a hard-fought 1-1 draw.
St Etienne striker Aubameyang cancelled out Admir Mehmedi's twice-taken penalty with a neat finish as the half-time whistle approached.
Belarus marked their first appearance at a major tournament since gaining independence 21 years ago with a 1-0 victory over New Zealand in an Olympic Games Group C clash at the City of Coventry Stadium.