Arsenal slumped to a first defeat of the season on Tuesday as Lens came from behind to win a pulsating Champions League clash 2-1 in France thanks to a fine second-half strike by Elye Wahi.
Gabriel Jesus gave the Premier League side an ideal start when he opened the scoring in the 14th minute to silence a raucous Stade Bollaert.
But Adrien Thomasson soon brought last season's Ligue 1 runners-up back on level terms and the visitors saw Bukayo Saka limp off before half-time.
Wahi, the 20-year-old striker who became Lens' club-record signing when he joined at the start of the campaign, then fired home in the 69th minute and his side held on for a famous victory.
"I am very proud of the players," said Lens manager Franck Haise, who took over the team in the second division in 2020.
"To beat Arsenal certainly makes it one of the most memorable games in my time at Lens."
The result lifts them above Arsenal to the top of Champions League Group B on four points, one above Mikel Arteta's side.
Arteta was left to rue the injury to Saka as he dismissed suggestions his side's travel disruption on Monday impacted their performance.
"He tried to back-heel a ball in the first half and he felt something, it was something muscular. He was uncomfortable to carry on so we had to take him off," said the Spaniard.
It was a miserable trip all round for Arsenal, whose flight to France out of London was delayed by almost five hours on Monday due to bad weather.
"There are no excuses," Arteta insisted.
"First of all congratulations to Lens. I think they are a really good side, really well coached. We knew it would be a really tough match."
While Arsenal are back in the Champions League after six seasons away, Lens are appearing in Europe's elite club competition for the first time in over 20 years.
That made their first European home game of the campaign all the more special, especially as it came against a team Lens famously beat in the Champions League back in 1998.
Arsene Wenger's Gunners had been the clear favourites against Lens then and Arteta's team were here, as they arrived in northern France unbeaten this season.
They underlined their status as the leading contenders in the group with a 4-0 demolition of PSV Eindhoven in their opening game, while Lens took heart from their performance in drawing 1-1 away to Sevilla.
Saka was deemed fit to start despite having limped off in Arsenal's 4-0 win at Bournemouth at the weekend, and his presence was vital as they went ahead early on.
- Saka injury blow -
They had not mustered a shot until Saka pounced on a loose ball back towards his own goal from Thomasson. Saka fed Jesus and he finished clinically.
It was a harsh lesson for Lens on what can happen if you make the slightest mistake at this level, but they capitalised on an Arsenal error to draw level on 25 minutes.
David Raya, again preferred to Aaron Ramsdale in goal, was out of his box when he tried to flight a pass out to the Arsenal right. The pass was intercepted and sent forward to Wahi, who did brilliantly to control the ball and lay it off for Thomasson to score with a magnificent strike.
Another setback followed for Arsenal as Saka went down just after the half-hour mark and was unable to carry on, with Fabio Vieira replacing him.
Both teams had chances shortly after half-time, but an even contest hinged on two remarkable moments around the midway point in the second half.
First Takehiro Tomiyasu met an Arsenal corner first-time only to be denied by a stunning instinctive save from Brice Samba, and then Lens took the lead three minutes later.
Przemyslaw Frankowski crossed from the right flank for Wahi, and the young striker swept a first-time shot into the far corner to make it 2-1 and lift the roof off the stadium.
The home side then withstood an Arsenal assault in the final minutes, with one heroic block from Jonathan Gradit summing up their performance.
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