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BELGRADE, Serbia (AP) — Skirmishes on Saturday erupted in Serbia between police and anti-government demonstrators who blocked roads and bridges in the Balkan country in protest against new laws they say favor interests of foreign investors devastating the environment.

Hundreds of people on Saturday appeared simultaneously in the capital Belgrade, the northern city of Novi Sad and other locations to block main bridges and roads for one hour in what organizers described as a warning blockade. They pledged further protests if the laws on property expropriation and referendum weren’t withdrawn.

Police officers blocked the demonstrators from reaching the bridges, which led to skirmishes as police helicopters flew overhead. The protesters then marched around while managing to stop traffic at a key bridge in Belgrade and in various central streets.

Organizers said a number of people have been detained. Police earlier have warned that any blockade of bridges is illegal.

A number of environmental groups and civil society organizations are angry that the authorities have lowered the referendum threshold and allowed for swift expropriation of private property if deemed to be in the public interest. Activists argue this will pave the way for foreign companies to circumvent popular discontent over projects such as the bid by the Rio Tinto company to launch a lithium mine in western Serbia.

Serbia’s authorities have rejected the accusations, saying the new laws are needed because of infrastructure projects. The country’s autocratic president, Aleksandar Vucic, said a referendum will be organized on the Rio Tinto mine.

Environmental issues recently have drawn public attention as local activists accuse the populist government of allowing for the devastation of nature for profit. Experts have warned that the planned lithium mine in western Serbia would destroy farmland and pollute the waters.

Following decades of neglect, Serbia has faced major environmental problems such as air and water pollution, poor waste management and other issues. Serbia is a candidate nation for European Union entry, but little so far has been achieved with regards to improving the country’s environmental situation.

Protesters on Saturday blew whistles during the blockade and chanted “We won’t give up Serbia.” Huge columns of cars and other vehicles formed at several locations as the demonstrators allowed only the emergency services to pass.

The protest coincided with a convention of Vucic’s populist Serbian Progressive Party as thousands of his supporters were bused into the capital for the gathering that was designed as a show of support for his policies.

Although formally seeking EU membership, Vucic has refused to align the country’s foreign policies with the 27-nation bloc and has instead strengthened the Balkan country’s alliance with Russia and China.