More than 70 homes lost power after the crash affected a power cable in Rhondda Cynon Taf.
Firefighters worked to free people trapped at Trebanog Road in Porth at about 09:00 BST on Wednesday.
Two people, a man and a woman who were in separate vehicles, are in a critical condition in hospital, according to South Wales Police.
The force said the other casualties, including the driver of the coach, needed hospital treatment but their injuries were not thought to be serious.
The Welsh Ambulance Service said it took three people to hospital – two to University Hospital of Wales, Cardiff, and one to Royal Glamorgan Hospital in Pontyclun.
Three people were freed from the wreckage while a further two managed to get out themselves.
The road is expected to stay shut « for some time » and South Wales Police asked people to avoid the area.
Measures to stop households mixing and early pub closures are expected to be introduced on Friday.
Porth, Wales, Rhondda Cynon Taff
Actu monde – GB – Bus crash injures five and causes power cut