Two 12-year-old boys were left distraught when their bikes were stolen from Chorley Library.
Friends David Pennington and Lewis Seed left their bikes inside the entrance to the library, and checked on them every five minutes when they visited last Wednesday.
They were using the computers, but after about half an hour, the bikes had disappeared.
David’s mum Lisa McDermott, of Buttermere Avenue, said: “David was so upset. It’s the second time he’s had a bike stolen in three months.
“The last one was taken from outside our house in April, and it was his birthday a few weeks later so we got him another one.
“He’d initially wanted a TV, but he uses his bike so much that he couldn’t do without one.
“It cost us £220 and he used it to ride all around Chorley. He goes to boxing and football on his bike, and visits family members too.
“I can’t believe it’s happened again. I don’t know what he’s going to do now.”
The Diamond Back Joker BMX is black and illuminous green, and was taken on Wednesday afternoon.
Lewis’s mum Julie Leigh, of Derwent Road, added: “Lewis wanted a new bike so much, he sold his old one and even his Xbox a few weeks ago to pay for it.
“He’d been saving up for ages, and it cost him £270 of his own money.
“He’s really gutted that it’s been stolen.”
Lewis’s bike is a black and white Haro BMX.
Police say bikes are stolen from the town centre because thieves take certain parts to use on their own bikes.
Community Beat Manager Nichola Barff-Lewis said: “We’ve found stolen bikes in the past, but they tend to have different handlebars or wheels on the original frames.
“Chorley Library is a bit of a hotspot for bike thefts, as is the bus interchange, and it might get worse in the summer holidays.”
Police have arranged to meet with children outside the library on August 6 to carry out ‘bike marking’, where they print the child’s house number and postcode onto the side of the bike.
It can only be read using a special fluorescent light, so if any stolen bikes are found, police will be able to identify who it belongs to.
“We’ll also be giving advice about how to keep bikes secure,” CBM Barff-Lewis added. We really urge children to lock up their bikes and not leave them on the streets.”