Tens of thousands of workers demonstrated Wednesday in the heart of Argentina's capital, and other regions as the country's largest union launched a 12-hour strike against tough economic austerity measures and reforms by new libertarian President Javier Milei.

The action, hitting sectors from transport to banks, is the biggest show of opposition to Milei's plans for spending cuts and privatization since he took office last month, pledging to fix an economy reeling from 211% inflation and crippling debt.

The strike was coordinated by the powerful umbrella union the General Confederation of Labor (CGT) and comes amid major scrutiny of Milei's two major reform pushes: an "omnibus" bill going through Congress and a "mega-decree" deregulating the economy.

"The first cut this government is making is to the workers," Pablo Moyano, leader of the powerful truckers union, said at the main union event in downtown Buenos Aires. "Their labor reform aims to take away workers' rights."

But even as the strikes, which started at noon local time, took a toll on transport, banks, hospitals and public services, Milei's government vowed to stick to its reform plans.

Local airlines said they had been forced to cancel hundreds of flights.

The CGT had already used the courts to temporarily suspend some measures relating to labor in Milei's decree.

The omnibus bill was approved by a committee and opened a new tab in the lower chamber of deputies early on Wednesday, one of many steps as it works its way through a divided Congress. It faces opposition from the powerful Peronist opposition bloc.

Milei, an economist and former TV pundit who pulled off a shock election win last year, is balancing stabilizing the South American country's economy and reducing a deep fiscal deficit with triple-digit inflation and with two-fifths of the population living in poverty.

The new government says the austerity measures are needed after years of over-spending that has left Argentina with huge debts to local and international creditors, including a shaky $44 billion deal with the International Monetary Fund (IMF).

"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, formerly Twitter.

"It's mafia unionists, poverty managers, complicit judges and corrupt politicians, all defending their privileges, resisting the change that society chose democratically."