Masked and helmeted protesters hurled stones and petrol bombs at riot cops who fired tear gas, water cannon and buckshot as demonstrators tried to advance toward the state ombudsman's office in central Caracas.
Maduro's political opponents vowed earlier to step up protests over his plan to reform the constitution, which they say is a bid to cling to power.
Monday's rally was the first since the weekend announcement of the stepping up of pressure on Maduro, although the MUD opposition coalition has not given details on what this would entail.
As in other rallies, demonstrators threw rocks and Molotov cocktails at police as they tried to march along a major highway in Caracas.
Opposition leader Henrique Capriles said he was beaten by National Guard as he left a rally broken up by tear gas.
And lawmaker Carlos Paparoni said he was injured in the head with tear gas canisters.
Other demonstrators said they were hit by buckshot fired by police.
Anti-government violence spread to other cities. In San Cristobal in the western state of Tachira, two taxis and a bus were set on fire and used to block a highway.
Supporters of Maduro marched in another part of Caracas.
Maduro has launched steps to set up a constituent assembly that the opposition says he plans to stack in his favor.
"If we allow the fraud that they want to call a constituent assembly, Venezuela will be lost," said Freddy Guevara, opposition deputy speaker of the National Assembly legislature.
He called on opposition supporters to "get ready for an escalation" of protests, but urged demonstrators to refrain from violence.
The opposition brand the leftist president a dictator and blame him for shortages of food and medicines. He accuses them of attempting a "coup" with US backing.
Prosecutors say 59 people have been killed in clashes since the protests erupted on April 1.
The government and opposition accuse each other of sending armed groups to sow violence in the protests.
