Chelsea's Champions League campaign was thrown back into the melting pot on their return to European action as chaotic defending and an error by keeper Ed de Goey allowed Marseille to clinch a wholly-deserved victory.

De Goey, normally such a reliable figure, remained rooted to the spot as he allowed the inspired Robert Pires to squeeze a shot past him from the tightest of angles after just 16 minutes

It may have been only the fifth goal that Chelsea had conceded in 11th Champions League games this season, including the qualifying games against Skonto Riga.

However, their first-half display was blighted by the sort of disorganised defending which so far this season only affected their Premiership campaign before Christmas but returned to haunt where they had previously been at their strongest - in Europe.

Indeed, it was only away to Hertha Berlin that Chelsea had previously struggled so much to impose themselves on a European game.

And while they entered this match at the top of their group with four points - whereas Marseille previously had none at all - they ended it needing to beat the French side back at Stamford Bridge next week to reignite their challenge.

The unexpected result from Rome, where Lazio succumbed 2-1 at home to Feyenoord, further ensured that any of the four sides could still qualify as the Dutch side now stand top, while Chelsea are still second.

To make matters worse though, there was the unedifying sight of riot police letting off tear gas in the enclosure containing Chelsea fans, even if it was difficult to immediately assign blame, with reports of missiles being thrown by the Marseille fans.

And Chelsea boss Gianluca Vialli, who was banished to the directors' box following his touchline dismissal last December against Lazio, certainly knows that his side cannot afford such a poor display again, even though they came close to equalising at the end.

Vialli, who had kept faith in youngster Jon Harley at left-back and dropped Celestine Babayaro, who was late back from the African Nations' Cup, from even the bench, was relying on mobile phone contact with assistant Graham Rix and coach Ray Wilkins.

However, in a stadium which was noisy if only two-thirds full at best, Chelsea were immediately in trouble against the French side, who beat Manchester United in the first group stage, even if they have been weakened by injuries and transfers since then.

Former Everton striker Ibrahima Bakayoko escaped down the right-flank past Harley and crossed to leave Cyrille Pouget with an open goal ahead of him.

However, Pouget, who had left the static figure of Frank Leboeuf trailing in his wake, stabbed his shot weakly and the ball dribbled harmlessly wide.

Chelsea steadied themselves briefly, with Tore Andre Flo being their main menace up front as he shot into the side-netting.

Then Harley showed his promise as he stormed down the wing and crossed deep for Flo to knock the ball down to Gianfranco Zola yet the Italian could not keep his volley on target from eight yards out.

Rix was soon on the touchline, shouting instructions to his side, yet Chelsea were still being undone at the back and Marseille duly took full advantage.

Midfielder Jerome Leroy lofted a through-ball over the head of Harley, who was straying out of position, and Pires went past Marcel Desailly as the centre-back seemed afraid to make a challenge just within the penalty area.

However, the angle was still tight and even though Pires' ensuing drive was powerfully struck, keeper de Goey should not have left such a large gap at the near post, allowing the ball to squeeze in off the upright.

In contrast, the Dutchman's opposite number, Stephane Trevisan, dived full-length to tip a shot from Flo around the post.

Chelsea's problems continued as a deflected cross then fell into the path of Pires, who was completely unmarked, and only Harley's last-ditch recovery tackle saved the visitors.

While Dennis Wise and Didier Deschamps were being completely over-run in midfield by Marseille's central trio, defenders Leboeuf and Desailly - who won the European Cup with the French side - were similarly disorganised.

There were acres of space on offer due to Marseille's intelligent movement and the fact that Gustavo Poyet was being forced inside from the left flank, but when Chelsea counter-attacked, they were all too often caught offside.

Zola did send one effort over the bar yet then the focus worryingly turned from the pitch to the terracing, where riot police let off tear gas in the Chelsea enclosure after missiles were reportedly thrown between the rival sets of fans.

The situation calmed down after the interval yet Chelsea were struggling to claw their way back into the game and, after Wise's claim for a penalty was waved away, the ineffective Deschamps was replaced by Roberto di Matteo.

The Italian, who had been out for the past seven matches with a broken arm, managed one shot on target while Flo volleyed over the bar and substitute Chris Sutton also forced a good save from keeper Trevisan.

However, although Chelsea certainly pressed forward with increased urgency, with one unseemly shoving match between the two teams, they flattered to deceive and midfielder Jerome Leroy troubled them from another tight angle as he shot into the side-netting.

At least Pouget's shot past de Goey was correctly ruled out for offside but even when Poyet was left with a free header from a corner in the final minute, keeper Trevisan pulled off a fantastic reaction save.

It was simply not Chelsea's night - and they cannot afford another weak display like this one if they are to progress to the quarter-finals.

Teams

Marseille: Trevisan, Cyprien, Luccin, Abardonado, Blondeau, Dalmat, Brando, Pires, Leroy, Pouget (De La Pena 72), Bakayoko (Moses 90).

Subs Not Used: Carasso, Issa, Martin, Keita, Martini.

Goals: Pires 16.

Chelsea: De Goey, Harley, Desailly, Leboeuf, Ferrer (Morris 81), Poyet, Deschamps (Di Matteo 62), Wise, Petrescu, Flo (Sutton 81), Zola.

Subs Not Used: Cudicini, Ambrosetti, Lambourde, Thome.

Att: 30,000

Ref: Rune Pedersen (Norway).