JUBA (Reuters) - South Sudanese police fired live ammunition at students protesting against a suspected land-grab of school property in Juba, wounding two people including a teacher, witnesses said on Wednesday.

Police denied shooting at students or teachers but said they had fired in the air to control the protesters, who had burned building materials and thrown rocks at police.

Human rights groups often criticize South Sudan's security forces, composed mostly of former guerrilla soldiers, over rights abuses and have urged the newly-independent government to better control its police and army.

Four witnesses said police arrived at the Juba Day Secondary School in the capital on Wednesday morning and started beating students who were protesting against the construction of a clinic, the International Freedom Hospital.

The students, who say the land where the clinic is being built belongs to the school, threw stones at the police in response. A second group of police then arrived in a vehicle and shot in the air and at the students, the witnesses said.

"I was in the process of stopping students taking revenge. At that moment I felt the bullet hit my right side and I fell down," Alafi Michael, a temporary teacher, told Reuters as he lay in bed at the emergency ward of a Juba hospital.

"They were shooting from outside, through the windows, through the doors," he said, holding up an X-ray image he said showed a bullet in his leg, which was later removed.

Police spokesman James Monday said he was not aware of any casualties from the protest but the police would investigate.

"The police tried to release bullets into the air to disperse the students and contain the situation," he told Reuters by telephone. "The police cannot fire at students. This was not reported to me."

Suzie Eggrey, 19, said the police beat students and teachers with their weapons. She said her friend Agnes Dusman was shot in the leg while hiding in the staff office.

"They were shooting through the window. They shot them in the school office, not randomly," she told Reuters.

South Sudan split from Sudan last year after voting overwhelmingly for independence under a 2005 peace deal that ended decades of civil war.

(Reporting by Hereward Holland; Editing by Alexander Dziadosz, Editing by William Maclean)