Soak chickpeas overnight in water. Drain.
In a food processor, combine chickpeas, onion, garlic, parsley, cumin, coriander, salt, baking powder, and flour. Process until mixture is coarse.
Form mixture into small balls or patties.
Heat oil in a deep pan. Fry falafel until golden brown, about 3-4 minutes per side.
In a bowl, mix yogurt, grated cucumber, garlic, lemon juice, and salt.
Serve falafel with cucumber sauce.