Intruders brandishing sticks and dressed in red broke through the front gate and set off fireworks in the interior gardens of the building, AFP journalists at the scene said.
The government supporters reached as far as the corridors of the congressional building, striking and injuring at least three lawmakers. The attackers ordered journalists to stop filming and taking photographs and leave the premises.
Lawmaker Yajaira de Forero named three of her colleagues who she said were struck, including one who was taken away for medical treatment.
Tension is high in Venezuela after three months of anti-government protests that have seen 91 people killed in clashes with police.
Protesters blame President Nicolas Maduro for a desperate economic crisis. He says the chaos is the result of a US-backed capitalist conspiracy by the opposition.
The opposition-controlled legislature was holding a special session to mark independence day when the government supporters burst in.
Before the violence broke out on Wednesday, Maduro's vice-president Tareck El Aissami had made an impromptu appearance in the congress along with the head of the armed forces, Vladimir Padrino Lopez and ministers.
El Aissami made an address in which he called on supporters of Maduro to come to the legislature to show support for him. A crowd of Maduro supporters held a rally outside the building for several hours before breaking into the grounds.
Maduro retains the public backing of the military high command -- a key factor in keeping him in power, according to analysts.
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