According to the police, the truck driver lost control of the vehicle and rammed into a auto first, then hit an electricity pole after which it crashed into roadside commercial shops at the busy Putalapatu-Naidupet junction.
The farmers were gathered outside the main police station in the town of Yerpedu protesting against illegal sand mining in their area.
A local reporter of a regional newspaper who was at the police station lost his legs in the accident.
The truck was going from Srikalahasti to Tirupati. The driver and the cleaner of the truck fled the scene soon after the incident.
The police shifted the injured to the government hospital in Chittoor and the deceased were sent for post-mortem.
Police from Chittoor said as per the preliminary information, the truck was carrying heavy load of crushed stones and travelling at high speed, hence lost control and ran over at least 20 persons.
Some police officers also sustained injuries, said Superintendent of Police (Tirupati Urban) Jayalakshmi. The police believe 10 people died on the spot while two died on the way to a hospital.
Meanwhile, Andhra Pradesh chief minister Nara Chandrababu Naidu expressing shock over the accident has directed his cabinet ministers Narayana and Amaranth Reddy to rush to the spot and coordinate relief operations.