The fistfight broke out after opposition lawmakers unfurled a banner in the National Assembly reading "Parliamentary Coup" and used whistles and vuvuzelas to protest a measure passed earlier that denied lawmakers the right to speak until they recognized Nicolas Maduro as president, Venezuela's El Universal newspaper reported.
Independent Venezuelan TV station Globovision obtained a short video clip showing assembly members hitting each other.
Assembly member Julio Borges later appeared on Globovision with a black eye, other facial bruises and blood running down his swollen cheek.
"They can beat us, jail us, kill us, but we will not sell out our principles," Borges said.
"These blows to Venezuela do not hurt me. The physical shock from the blows gives strength to me and my colleagues to keep fighting for a better future," he said.
Pro-government legislators appeared on state TV accusing "fascist" opposition lawmakers of attacking them.
The opposition said more than a dozen of its allies and a half-dozen pro-government lawmakers were injured. Lawmakers from the ruling United Socialist Party did not immediately say how many deputies were injured.
Maduro -- a 50-year-old former bus driver and union activist with close connections to the Castro brothers in Cuba -- was declared the winner of Venezuela's April 14 election to succeed the late leader Hugo Chavez.
The National Electoral Council, responsible for overseeing and guaranteeing the transparency of all elections, said Maduro defeated opposition candidate Henrique Capriles by less than 2 percentage points.
Capriles -- a 40-year-old law school graduate and governor of Miranda, Venezuela's second-most-populous state -- called Maduro's slim win "illegitimate," citing thousands of irregularities, including thousands of dead people on the voting rolls, and demanded a recount, a call also made by the Organization of American States.
He insisted the recount include paper validations of the votes.
The electoral council refused to do the full recount, completing an electronic recount instead.
That recount, which included votes cast abroad, reduced Maduro's victory margin to 1.49 percent from 1.8 percent. The margin represents fewer than 225,000 votes out of 14.8 million cast.
The government and opposition planned rival marches in Caracas Wednesday to commemorate International Workers' Day, also known as May Day, celebrating the labor movement.
Both sides asked marchers to remain peaceful.