BUENOS AIRES, Argentina — Argentina's biggest trade unions mounted one of their fiercest challenges to the libertarian government of President Javier Milei, staging a mass general strike on Thursday that led to the cancellation of hundreds of flights and halted key bus, rail and subway lines.

Main avenues and streets, as well as major transportation terminals were left eerily empty. Most teachers couldn't make it to school and parents kept their children at home. Trash collectors walked off the job — as did health workers, except for those in emergency rooms.

The 24-hour strike against Milei's painful austerity measures and contentious deregulation push threatened to bring the nation of 46 million to a standstill as banks, businesses and state agencies also closed in protest.

Thursday's action marked the second nationwide union strike since Milei came to power last December, slashing spending, laying off government workers, and freezing all public works projects in a bid to rescue Argentina from its worst financial crisis in two decades.

He has also devalued the local currency, stabilizing the peso but also causing prices to soar. Argentina's annual inflation rate now nears 300% — considered the highest in the world, outpacing even crisis-stricken Lebanon.

The government said transport service disruptions would prevent some 6.6 million people from making it to work. During the morning rush-hour on Thursday, few cars could be seen on streets typically snarled with traffic. Garbage was already piling up on deserted sidewalks.

Milei posted a photo on Instagram holding up a soccer jersey emblazoned with the words ''I DON'T STOP.''

The country's largest union, known by its acronym CGT, said it was staging the strike alongside other labor syndicates ''in defense of democracy, labor rights and a living wage.''

Argentina's powerful unions — backed by Argentina's left-leaning Peronist parties that have dominated national politics for decades — have led the pushback to Milei's policies on the streets and in the courts in recent months.

''We are facing a government that promotes the elimination of labor and social rights,'' the unions said, seeking to portray Thursday's strike as an eruption of public outrage over Milei's free-market policies that have disproportionately affected poor and middle classes.

The government downplayed the disruption as a cynical ploy by its left-wing political opponents.

''They want to keep Argentina on a path of servitude,'' said presidential spokesperson Manual Adorni of the union leaders, accusing them of ''extorting Argentines to try to return to power.''

Union leaders said they had no choice but to escalate their actions after Argentina's lower house approved Milei's state overhaul bill and tax packages last week.

Even as lawmakers scrapped the bill's most controversial articles, unions remain vehemently opposed to parts of the package that relax labor market regulations and grant Milei power to restructure and privatize public agencies. The bill is now being debated in the opposition-dominated Senate.

Rubén Sobrero, general-secretary of the Railway Union, said the unions were prepared to extend the strike if negotiations did not yield results. ''If there is no response within these 24 hours, we'll do another 36,'' he said.

For months, most recently Monday and Tuesday this week, raucous demonstrations by leftist parties gripped Buenos Aires, the country's capital — in sharp contrast to the silence prevailing on the streets Thursday.

Argentina's main international airport warned travelers to check in with their airlines as flight boards in terminals displayed a stream of yellow cancellation notices. The country's flagship carrier, Aerolíneas Argentinas, announced it had canceled nearly 200 domestic and regional flights and rescheduled over a dozen international flights, affecting 24,000 passengers and costing the airline $2 million.

Only one bus company said it would continue regular service on Thursday. Shortly after the strike began at midnight, police said protesters attacked two of the company's buses in Buenos Aires, breaking windows but causing no casualties.

''We won't let them (the unions) break everything we are achieving,'' conservative Security Minister Patricia Bullrich posted on social media with a photo of the shattered bus windows.

Experts say that both sides are politically motivated.

In using the strike to assail his rivals, Milei is compensating for worsening economic pressures, said Sebastián Mazzuca, a political science expert at Johns Hopkins University. In bringing the economy to a halt, unions that had their candidate defeated in the last presidential election are flexing their muscles.

''This conflict is sold to the public as a social conflict, but it's really a political conflict,'' Mazzuca said. ''The outgoing government doesn't want to die. And the new government wants to stay in power.''