Place chickpeas in a large bowl and fill with cold water until they’re covered. Let soak overnight or for 12 hours, then drain well.
Move chickpeas to a food processor fitted with a steel blade and process until ground.
Take the slice of bread, soak it in water, then squeeze out the excess. Crumble it into pieces in the food processor. Add all remaining ingredients and process until mixture is combined but slightly granular.
Form mixture into balls, about the size of a walnut (they’ll expand during frying).
Frying: In a large, deep, heavy-bottomed saucepan, heat 2-3 in./5-7 cm of oil. To test if the oil is hot enough, insert a thermometer into the oil and see if it reaches 375F/190C. If you don’t have a thermometer to test, insert a wooden spoon into the oil; it’s ready for frying when bubbles appear around the stick. If it bubbles vigorously, it’s too hot. Carefully add falafel balls, a few at a time, depending on how large the saucepan is. If falafel falls apart, you may need to add more flour to a too-wet mixture, or turn up the heat. Fry until golden brown, a few minutes on each side. Drain and move to a paper towel-lined plate.