The "superclasico" was delayed more than two hours after Boca players were affected by tear gas and broken glass after their team bus was pelted with "pepper spray, sticks and stones" on the way to the stadium in Buenos Aires.
Despite reluctance from Boca players and staff, tournament organizers Conmebol, the South American football governing body, said the match between the local arch rivals at River's Monumental stadium would kick-off at 7.15pm (2215 GMT), more than two hours after its scheduled start time.
"We're not in a condition to play," complained Boca's veteran forward Carlos Tevez, formerly of Manchester United and City, on Argentine television.
"They're obliging us to play," added the 34-year-old, a Copa Libertadores winner with Boca in his first of three spells at the club in 2003.
Conmebol had earlier delayed the match from 5pm to 6pm due to the bus attack. Local television stations had reported Boca were pushing for the game to be called off.
Boca players were left coughing and teary eyed after the attack that shattered several of their bus's windows.
'WE CAN'T PLAY'
"They attacked us from everywhere," said Boca captain Pablo Perez, who was taken to a local hospital for treatment before returning to the ground.
"Pablo has just returned from hospital and has a bandage on his eye," added Tevez.
"Other team-mates have cuts. We've only just been able to breathe well because we were affected by the gas. We can't play like this," said Tevez, a Champions League winner with Manchester United in 2008.
A statement from Conmebol's medical commission shown on television said some players had suffered "superficial cuts" and that there was no medical reason to postpone the match.
The attack came despite the bus travelling with heavy police protection ahead of clash between the two most popular teams in the country -- dubbed the biggest club match in the history of Argentine football.
"They threw pepper spray, sticks and stones. It came in the bus from everywhere," said central defender Carlos Izquierdoz.
Television pictures showed Boca players being attended to by medical staff in their dressing room at the stadium in the upmarket Nunez neighborhood of Buenos Aires.
'OVERWHELMED BY VANDALISM'
A Boca official told the press that the pepper spray came from the police trying to disperse River fans.
"There was a lack of control in the last streets close to the Monumental. The police were overwhelmed by all the vandalism and had to disperse the River fans with tear gas," said the unnamed official.
"Because of the wind and the broken windows, it entered the bus."
The final of the Copa Libertadores, the equivalent of Europe's Champions League, is finely poised at 2-2 following the first leg at Boca's Bombonera stadium two weeks ago.
It is the first time ever it has been contested by two Argentine sides.
There will be no Boca fans at the second leg as travelling support has been banned at all Argentine football matches since 2013 due to hooliganism.
Football-related violence has cost the lives of more than 300 fans in Argentina over the last 50 years, according to figures from the Salvemos al Futbol (Let's save football) charity.
This local encounter is widely viewed as the fiercest rivalry in world football.
River have won the Copa Libertadores three times, the last of those coming in 2015.
Boca are six-time winners, a record bettered only by fellow Argentines Independiente with seven, but last lifted the trophy more than a decade ago.