The pop singer posted some alarming photos Tuesday showing himself in a hospital bed, and recovering at home ... and he told fans he got banged up when his Vespa was struck by another vehicle last week.

Paul says he was on his way home from the gym on his moped ... and the next thing he remembers is waking up on a stretcher as he was loaded into an ambulance. He has no recollection of the collision, but is grateful to be alive.

Sources familiar with the situation tell TMZ ... the accident happened last Thursday in L.A., and Paul was on La Brea Ave when the car turned into him and knocked him unconscious. He was taken to Cedars-Sinai, and doctors released him Friday in a wheelchair.

Sources say he's back on his feet now, but is using crutches to get around ... as he's still very sore and bruised.

Paul expressed a similar sentiment on Instagram, adding ... "all of my vitals are good, but walking and moving around is still a challenge. I’m getting progressively better though and will be at 100% really soon."

Nonetheless, PJK noted his doctors have advised him to take it easy for a while -- so, he's been forced to cancel upcoming shows in Australia and New Zealand.

The band was originally scheduled to perform 5 days next week in Australia, and then one more in New Zealand on June 30.

Paul added, "thank you for understanding. this experience has only intensified the feelings of purpose i have, so i can’t wait to heal up and get back to it. all other tour dates are staying the same. i love you and this life deeply! see u soon."

Paul's famous friends and fans have since sent him well wishes -- with 'Queer Eye' star Antoni Porowski writing in the comments ... "sending you all the love brother 🙏🏼 so relieved it wasn’t worse."