Police vans and patrol cars, as well as a number of paramedic vehicles, rushed to Crosby Marina Park at around 5.45pm yesterday.
A group of teenage girls who had been helping the boy were left covered in blood after the incident.
Officers sealed off an area of sand dunes off South Road in Waterloo and were seen scouring the area looking for clues.
Onlookers said the beach had been packed with locals and tourists enjoying the sunshine and sizzling temperatures.
One witness said: 'I was told there were teenage girls covered in blood who said their mate, a 14-year-old boy, had been stabbed quite badly.
'He's apparently not in a good way and had passed out. The girls were very distressed and waiting for their parents to pick them up.'
Another visitor to the beach said: 'We had taken the kids out for the afternoon but police wouldn't let us past.
'People are saying youths were involved and someone had been stabbed. We take the children here most weekends and I've never seen this many police here.'
Merseyside Police confirmed a boy had been stabbed.
Detective Inspector Paul Parry said: 'We initially believe that this was a targeted attack, and one which thankfully appears to have resulted in minor injuries.