Two of pop music's biggest stars have cancelled concerts, with Canadian sensation Justin Bieber axing a show in Portugal and Rihanna the second night of her just-launched tour.

Bieber's Tuesday night concert in Lisbon has been cancelled "due to unforeseen circumstances," according to a statement on the concert venue's website. It was to have been the second of a two-night stint in the Portuguese capital.

"The Canadian singer is eager to play for the Portuguese fans" at Monday night's concert, organizers said.

Though no specific reason was given for eliminating the second date, Portuguese media have reported slower sales for Tuesday's show. Ticketholders were directed to seek refunds.

The news comes after a tough week for the 19-year-old Stratford, Ont., singer, who celebrated his birthday in London on March 1.

On Friday, Bieber was captured on camera in the midst of an expletive-filled exchange with paparazzi.

The shouting match occurred the day after he collapsed, short of breath, mid-concert. He required oxygen as well as a quick check-up in hospital after he had finished the show.

Just days before that, he sparked the ire of some young British fans — as well as their parents — when he showed up onstage much later than expected for the first of his O2 Arena shows.

"Too many blessings to allow the stress in," Bieber told fans on Saturday via Twitter, where he has more than 35 million followers.

"The press made things look nuts, but really I have enjoyed London. The fans are incredible."

Rihanna plagued by laryngitis

Barbadian-born Rihanna kicked off her Diamonds tour with a sold-out concert in Buffalo, N.Y., on Friday.

Laryngitis forced her to cancel her Sunday night concert in Boston, the second night of her tour. Doctors ordered the pop star to rest her inflamed larynx, organizers said.

The axed show will be rescheduled, while Rihanna is slated to resume her busy tour in Baltimore on Tuesday.

"I hate disappointing people," the singer posted online.

"I'm so embarrassed about this! Thank you for your prayers and well wishes."