It was the latest eruption of political violence in three months of chaos in the oil-rich, poverty-stricken South American country.
The attackers were supporting President Nicolas Maduro against opposition demands for elections to remove him from office.
Military police guarding the site stood by as about 100 intruders brandishing sticks and pipes broke through the front gate of the National Assembly and into the interior gardens.
The mob reached as far as the corridors of the congressional building, striking and injuring lawmakers.
The attackers, one of them holding a gun, ordered journalists to leave the premises.
The speaker of the assembly, Julio Borges, in a Twitter message named five lawmakers who were hurt in the violence. Some were taken away for medical treatment.
"This does not hurt as much as seeing every day how we are losing our country," one of them, Armando Armas, told reporters as he got into an ambulance with his head wrapped in bloody bandages.
Deputies also said that two employees of the assembly were hurt.
'Savages'
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 government always resorts to violence," said opposition lawmaker Stalin Gonzalez after Wednesday's attack.
Another deputy, Simon Calzadilla, vowed: "We will continue to face up to these savages."
The opposition-controlled legislature was holding a special session to mark independence day when the government supporters burst in.
Earlier 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 the president.
A crowd of Maduro supporters held a rally outside the building for several hours before breaking into the grounds during a recess.
'Trojan horse'
Government and opposition have accused each other of using armed groups to sow violence.
The opposition has accused plainclothes armed pro-government groups of attacking protesters.
While the chaos erupted in the grounds of the assembly, Maduro was watching a military parade in another part of Caracas, with troops marching and armored vehicles rumbling by.
In a speech at the parade, he demanded that the military be "ever loyal, never traitors."
"Let no one become a Trojan horse," he said. "Let no one become a servant of the oligarchy."
Maduro retains the public backing of the military high command -- a key factor in keeping him in power, according to analysts.
But the president last month said he was replacing four other senior commanders of the armed forces.
Maduro has infuriated his opponents by launching a plan to form an assembly to rewrite the constitution.
Opponents say he will pack the "constituent assembly" with allies to cling to power.
Voting for members of the assembly is scheduled for July 30.
"However, the political crisis is so fluid that the country could reach a tipping point before then," wrote analysts from the Eurasia Group consultancy last week in a note.
The opposition on Monday said it would hold a popular vote on July 16 against the constitutional plan.
