BEIRUT, Aug 4 (Reuters) - A large explosion rocked Beirut on Tuesday, injuring many people as glass shattered and balconies collapsed from the impact, Reuters witnesses said.

Lebanon's state news agency NNA and two security sources said the blast had occurred in the port area where there are warehouses housing explosives. It was not immediately clear what caused the blast or what kind of explosives were in the warehouses.

"I saw a fireball and smoke billowing over Beirut. People were screaming and running, bleeding. Balconies were blown off buildings. Glass in high-rise buildings shattered and fell to the street," said a Reuters witness.

Local broadcaster LBC quoted the health minister as saying there was a "very high number" of injures and a large amount of damage. Al Mayadeen television said hundreds were wounded.

Another Reuters witness said she saw heavy grey smoke near the port area and then heard an explosion and saw flames of fire and black smoke: "All the downtown area windows are smashed and there are wounded people walking around. It is total chaos."

(Reporting by Samia Nakhoul and Yara Abi Nader; Additional reporting by Dubai and Beirut bureau; Writing by Lisa Barrington and Ghaida Ghantous; Editing by Gareth Jones and Edmund Blair)