The president has said that the austerity measures are due to years of overspending that have resulted in huge debts.
Wednesday’s demonstration is the most significant show of opposition to Milei’s spending cuts and privatisation plans since he took office last month and pledged to fix an economy dealing with 211 percent inflation.
The strike, coordinated by the umbrella union, the General Confederation of Labour (CGT), comes amid scrutiny of Milei’s two significant reforms: the “omnibus” bill going through Congress and a “mega-decree” deregulating the economy.
“Milei wants a country where poverty and informal work reaches 90 percent,” union member and national opposition deputy Hugo Yasky said on local radio station Radio Con Vos.
“Now there is no job creation. What there is now is widespread misery, people’s desperation, there are no measures to mitigate the damage they are causing.”
Earlier on Wednesday, the omnibus bill was approved by a committee in the lower congressional house, the Chamber of Deputies.
The mass strikes began at 12pm (15:00 GMT) and affected transportation, banks, hospitals, and public services.
Local airlines said they had been forced to cancel hundreds of fights due to the demonstration.
Protesters held placards that read “The homeland is not for sale” and “Eating is not a privilege” as some others held a giant puppet of Milei.
Another poster said, “Today’s retirees are yesterday’s workers, stop robbing them!”
Al Jazeera’s Lucia Newman, reporting from Buenos Aires, said it was “impossible” to determine the number of people attending the protest due to its scale.
“There seems to be a kind of unofficial agreement with the strikers and the security minister to allow these huge numbers of people to be here but only if they cannot disrupt traffic,” Newman said.
“It’s still very, very tense, and it’s an ongoing situation here, but it’s a huge turnout so far.”
Milei’s government said that the austerity measures are due to years of overspending that have left the South American country with huge debts to local and international creditors, including a $44bn deal with the International Monetary Fund.
“There is no strike that stops us, there is no threat that intimidates us,” Milei’s security minister and former presidential election rival Patricia Bullrich wrote on X.
“It’s mafia unionists, poverty managers, complicit judges and corrupt politicians, all defending their privileges, resisting the change that society chose democratically.”
Milei, an economist and former TV pundit, assumed the presidency after a shock win in last year’s general election.