Superintendent of Police Anand Kumar Singh said the mishap took place when many people, including children, climbed the wall to see the helicopter of Shah landing at Babui tola Sports Ground, the rally venue.
The wall was old and had developed cracks which collapsed under the heavy load of people, the SP said.
The accident made some people run helter skelter but the situation was brought under control by the police and civil officials.
Amit Shah expressed sympathy with those injured in the mishap.
He later addressed the rally and appealed to the people to ensure victory of NDA in Bihar.