One Direction singer Niall Horan has appealed to fans to "limit" the items they throw on stage.
The 20-year-old was doubled up in pain after he was struck on the knee by an unidentified object at the band's Amsterdam concert.
Video footage of the concert at the Amsterdam Arena showed Horan struggling to walk a short ramp with his electric guitar strapped across his shoulder following the incident.
The pop star was comforted by his friends and the hashtag "feelbetterNiall" began trending on Twitter.
After the show he tweeted: "Can you limit what gets thrown on stage please! Coz somethin thrown at me tonight! Hit my knee! A lot of pain from it!"
"Never wana sound like I'm complaining! But only having my operation 5 months ago, I still get a bit of pain and I'm very scared about it, he added.
Also guys! Can you limit what gets thrown on stage please! Coz somethin thrown at me tonight! Hit my knee! A lot of pain from it!
— Niall Horan (@NiallOfficial) June 25, 2014
Horan had surgery on his knee in the US in January to correct an old football injury.
The singer put the incident aside to thank the Dutch fans for "a great couple of shows."
While the Where We Are stadium tour moves on to Italy for a gig at Milan's San Siro Stadium on June 28, there have been reports that One Direction are to collaborate with Pharrell Williams.
After performing at Capital FM's Summertime Ball in London on Saturday, the singer told Sugarscape that he and Liam Payne have been spending some time together discussing music.
"Liam stopped by my studio," he revealed. "I think he's just genuinely curious about music. We have that in common. He's always wanting to see what's new and what's next."
The Happy singer said he would be glad to work with the boy band, adding: "But I suppose they already have five voices - I don't know if they need an extra one."
The One Direction tour heads to the US in August.
The band will perform a total of 67 dates across the globe for the Where We Are tour, which opened in Bogata, Peru.