Witnesses said lives were saved when an automatic rifle used by one of a number of gunmen jammed, but they claimed up to 15 shots were fired in the small premises in the New Lodge district. The most seriously injured was an elderly man hit in the chest.
Responsibility for the attack was claimed by the illegal Ulster Defence Association.
Afterwards local people said the RUC and Army should do more to prevent such shootings, pointing out that the attack took place about 100 yards from a police station. The area, one of Northern Ireland's most violent, is generally heavily patrolled by troops and police.
For the security forces the incident follows several reasonably successful months of activity against violent Protestants. Loyalist paramilitants have complained recently that they are the subject of heavy security force surveillance.
The attack was an attempt to emulate two indiscriminate attacks on Catholic-frequented betting shops in the city. In February last year, five men were killed and nine injured in a similar attack in south Belfast, while last November three were killed and 13 injured at another shop in north Belfast.
Yesterday's attack took place in bright sunshine as punters waited to watch a 3pm race. Witnesses said that when the gunmen opened fire those inside threw themselves on the floor.
Locals said the gunmen fired more shots as they made their getaway in a car which had been hijacked earlier in the loyalist Shankill Road area, less than half a mile away.
A woman said: 'When I got to the bookies there was pandemonium . . . Everybody was squealing. Somebody says to me, 'Mairead, it's your father, it's your father' just as they brought him out on a stretcher.'
The worst of the injured was described as being in a serious but stable condition.
A local priest, Father Francis McAllister, said: 'It's just terrible. There's no way you can guard against something like this. It was the time when schoolchildren were coming home so they could quite easily have been injured as well. People here are living in fear and terror. They're just totally shattered. It's very easy for anyone to come in and do a shooting and then make a quick escape.'
Fifteen people have been killed by loyalists so far this year. Earlier this week Peter Robinson, the Democratic Unionist MP for East Belfast, said the surge in loyalist violence was a backlash to government policies which were leading towards a united Ireland.