A 10-year-old boy had to be dragged along by a rope and pulled from a muddy riverbed after becoming trapped alongside his two friends.
Their eight-year-old companion raised the alarm when the boys became stuck while playing in the foreshore of the River Thames.
The eldest of the group was rescued after officers dropped a rope over a 30ft wall at Sunderland Point, south east London,having used thermal imaging technology from the police helicopter to find the boy, who was trapped up to his waist.
He was dragged for 25 yards before reaching steps from where he was rescued. His two friends, both aged eight, had managed to free themselves as police arrived at the foreshore, between Galleons Point and Woolwich Pier yesterday morning.
All three were taken to hospital as a precaution but were not injured, police said.
PC Richard Burn from the Marine Policing Unit said the boys were lucky the tide was out.
"Thankfully the tide was out, so rescuing the boy was difficult but ultimately successful," he said.
"If the tide had been coming in it might have been a different outcome. All the boys are safe now with their parents and are cold if not a little scared by (the) events.
"I would like to remind the public of the dangers of venturing onto the Thames foreshore when the tide is out. If you do, you may become stranded when the tide comes in or get stuck in dangerous mud."