Defence Minister Lindiwe Sisulu condemned in "the strongest terms possible" an illegal protest by about 3 000 soldiers who attempted to gain access to the Union Buildings in Pretoria on Wednesday.

"Anarchy and lawlessness have no place in the defence force... sheer acts of vandalism and threat to human lives by members of the South African National Defence Unions (Sandu), represents the worse form of criminality in a democracy," Sisulu said.

Sisulu said the High Court granted an interdict against the march, and soldiers were informed that it was illegal, but Sandu went ahead.

"Within the defence environment this lawlessness is a very serious breach of the nation's trust, especially those charged with the protection of state security. The chief of the South African National Defence Force has been asked to deal with this."

The Minister also asked the Minister of Police to act decisively and to ensure that the "full might off the law is brought to bear".

Earlier Tshwane police used rubber bullets and teargas to disperse the soldiers.

When a group of protesters tried to climb over the first fence leading onto the property, police retaliated with the "necessary force", Defence Department spokesperson Ndivhuwo Mabaya said.

"As the protesters were leaving they damaged some cars and set alight a military vehicle."

One protester was slightly injured.

The soldiers refused to leave the lawn of the Union Buildings on Wednesday despite a memorandum having been handed over to the area police commissioner.

"Sandu must take responsibility for this. As the Ministry of Defence and Military Veterans we are going to be sending Sandu legal documents with a matter of urgency."

He said damages to State property would also be claimed from the union.

"It [the protest] was illegal. Sandu has been irresponsible to mobilise and encourage their members."

Mabaya said the protesters had not been wearing uniform, so it could not be immediately ascertained if they were soldiers, but the process of determining who had not arrived for duty had already begun.

"If any soldier was involved we are going to take the necessary disciplinary procedures. In the military there are very aggressive steps taken, absent without leave is a very serious crime."