|Not the best way to start Inter's European challenge for glory - Diego Milito rues their costly loss [GALLO/GETTY]
Trabzonspor marked their Champions League debut by stunning Inter Milan 1-0 away on Wednesday while English giants Manchester United and Manchester City were held to draws as they failed to transfer their domestic prowess to the European stage.
Czech defender Ondrej Celustka scored the 76th-minute winner at San Siro for Trabzonspor, who only qualified for the competition because Turkish rivals Fenerbahce were removed while the club is investigated for match-fixing.
United, last season's runners-up, rallied to draw 1-1 at Benfica thanks to veteran winger Ryan Giggs' stunning equaliser while Aleksandar Kolarov's free kick earned City a 1-1 draw at home to Napoli in their first match at Europe's elite level since 1968.
Elsewhere, nine-time winners Real Madrid opened their group campaign with a 1-0 victory at Dinamo Zagreb despite playing the last 17 minutes a man down after Marcelo's dismissal.
German giants Bayern Munich, who have won the competition four times, won 2-0 at Villarreal to take the early lead in Group A, which also contains Napoli and City.
CSKA Moscow rallied from two goals down to draw 2-2 with French champions Lille, while Ajax were held 0-0 by Lyon and Basel beat Romanian debutante Otelul Galati 2-1.
Inter's defeat piles the pressure on under-fire coach Gian Piero Gasperini, whose team was also beaten 4-3 at Palermo in their Serie A opener on Sunday - conceding four goals for the first time in a league match in more than seven years.
"It's difficult, without a doubt,'' said Inter goalkeeper Julio Cesar.
"In a moment like this we all have to stay together, work hard and find our way out of this situation.''
Substitute Coutinho squandered Inter's best chance in the 88th minute, drawing one of a number of saves out of Trabzonspor goalkeeper Tolga Zengin.
In the other match in Group B, Lille powered into a two-goal lead by the 57th minute through striker Moussa Sow and midfielder Benoit Pedretti but was pegged back following a double by Seydou Doumbia, the Ivory Coast forward's second goal coming in the 90th minute.
Fierce rivals United and City have both won their opening four matches of the season in the Premier League, producing some breathtaking attacking football in the process, but they were brought back down to earth in Europe.
Oscar Cardozo's classy 24th-minute goal put Benfica ahead in the Group C match but the 37-year-old Giggs collected Antonio Valencia's pass, cut inside from the right and fired a left-foot drive from the edge of the area into the top corner for United's equaliser.
The visitors had goalkeeper Anders Lindegaard - filling in for the rested David De Gea - to thank for preserving a draw, with the Denmark international denying Nolito and Nicolas Gaitan late on.
Basel is the group's early leader, with Alexander Frei's 84th-minute penalty securing the Swiss side all three points. Fabian Frei put Basel ahead, only for Marius Pena to level in the 58th for Otelul Galati.
Big-spending Man City's expensively assembled team was given a reality check by Napoli, who marked their first game at Europe's highest level since 1990 - and the days of Argentina great Diego Maradona - by going ahead through Edinson Cavani in the 64th.
Kolarov's whipped free kick five minutes later salvaged a point for City, who had earlier hit the woodwork through Yaya Toure and Sergio Aguero.
Toni Kroos and substitute Rafinha were the goalscorers for Bayern Munich as the German powerhouse began their pursuit of a fifth European championship with a comfortable victory.
Angel Di Maria was the matchwinner for Real Madrid in the 53rd, the Argentina winger finishing off a flowing move that included Cristiano Ronaldo, Karim Benzema and Marcelo.
Brazil left back Marcelo was sent off in the 73rd when he earned a second yellow card for diving under Jerko Leko's challenge but Jose Mourinho's Madrid, playing in red jerseys for the first time in 38 years, held on.