BUENOS AIRES (Reuters) - Protesters in Argentina clashed with police during marches over government austerity measures on Thursday as labor unions challenged President Mauricio Macri in the first general strike since he took office 16 months ago.

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Security forces used high-powered water cannon and tear gas to control picketers who had blocked the Pan-American Highway, the main road leading from the north to capital city Buenos Aires, where normally bustling streets were half-empty and businesses were closed.

Truck and bus drivers, teachers, factory workers, airport employees and the government customs agents who run Argentina’s all-important grains export sector walked off the job at midnight for 24 hours.

“No customs officials are here, so there will be no exports or imports today,” said Guillermo Wade, manager of the maritime chamber at Argentina’s main grain hub of Rosario.

The country is the world’s top exporter of soymeal livestock feed and the third-largest supplier of soybeans.

Macri took office in December 2015. He eliminated currency and trade controls and cut government spending, including gas subsidies, a move that steeply pushed up home-heating bills.

The strike came as Macri welcomed hundreds of potential investors and foreign officials to a meeting of the World Economic Forum in Buenos Aires. Blocks away from the hotel where the meeting was held, protesters clamored for wage increases in line with inflation, which was 40 percent last year and expected to be about 20 percent in 2017.

“The situation is dramatic,” Julio Piumato, a spokesman for labor umbrella group CGT, said in a telephone interview.

“Wealth is being concentrated in the hands of a few at the same rate that poverty is growing,” he said. “Urgent measures are needed to create employment. One out of every three Argentines is poor.”

The one-day work stoppage came ahead of an October congressional election that will gauge Macri’s strength going into his 2019 re-election bid. The market is concerned about a political comeback by previous President Cristina Fernandez, who boosted the government’s role in the economy during her eight years in power.

Macri took office promising a wave of foreign investment that has been slow to manifest itself. Investors want to see that his Cambiemos coalition remains the biggest vote-getter in heavily populated areas like Buenos Aires, which will be key to the 2019 election.

He was elected after Fernandez left Argentina with rampant inflation, dwindling central bank reserves and a wide fiscal deficit.