PANAMA (AP) — Protesters blocked streets across Panama on Monday, demanding the government rescind a contract to continue copper mining in a biodiverse region.

Teaching and construction unions led calls against the contract with environmentalists, saying continued development threatens forested land and crucial groundwater just 75 miles (120 kilometers) west of the capital, in the state of Colon.

Across Panama City, peaceful protesters handed out fliers, but in some areas on the outskirts of the capital police met protesters with tear gas. In anticipation of the largest marches since a cost of living crisis last July, both the Department of Education and the University of Panama cancelled classes.

The government used social media to highlight the “enormous contribution” the mine — Panama’s largest private investment ever — makes to the country’s economy.

In March, Panama’s legislature reached an agreement with Canadian mining company First Quantum, allowing its local subsidiary, Minera Panama, to continue operating a huge open-pit copper mine in central Panama for at least 20 more years. The mine was temporarily closed last year when talks between the government and First Quantum broke down over payments the government wanted to receive.

Protests began after President Laurentino Cortizo signed off on that contract on Friday after it was approved by the congress.

The president acted surreptitiously, according to Fernando Abrego, leader of Panama’s Association of Teachers union. “The government decided this confrontation by quickly and expeditiously approving a contract they know is rejected by the people,” he said.

Teachers were joined by construction workers, who are one of country’s most powerful labor groups. “The people are in the streets in defense of sovereignty,” said union leader Saúl Méndez, “in the face of a contract that cedes self-determination by devastating the environment to steal resources.”

It is unclear how persuasive these arguments will prove against the economic boon of a single mining site that already brings in 3% of the nation’s gross domestic product. Minera Panama says the mine will employ thousands of Panamanians and that its shipments make 80% of the country’s total exports.

The new contract, initially slowed by labor disagreements, secures Panama at least $375 million a year from Minera Panama, over 10 times more than the previous deal. It represents one of the largest national mining contracts in a region where other countries like Costa Rica regulate the sector more stringently and El Salvador which banned metal mining in 2017.

For teachers, however, Abrego said concession was not an option, and that the teachers’ union would hold an assembly to plan their next actions. “We will remain in the streets,” he said.