The protests have become a headache for Serbia's ruling coalition ahead of an April 3 general election, and Prime Minister Ana Brnabic told Reuters this week a decision on whether to let the project proceed would be made after the vote.

The government has offered mineral resources to foreign investors including China's Zijin copper miner and Rio Tinto as it seeks to boost economic growth, but green activists say the mining projects will cause pollution.

"We do not want to be quiet ... We want this country to be (safe) for our children," said Ivana, asking not to give her full name, who was among a group of protesters blocking a highway in a neighbourhood of the capital, Belgrade.

Many of the protesters carried banners. "Stop investors, save nature", one of the placards read.

A crossing point to neighbouring Bosnia was also blocked along with roads near the towns of Cacak and Sabac.

The anti-mining protests have been held every Saturday since late November, with a break for the New Year and Orthodox Christmas holidays.

Soon after the demonstrations began, conservative President Aleksandar Vucic asked parliament to rework an expropriation law allowing the state to swiftly acquire property for potential development. Environmentalists had opposed the legislation.

Serbia is one of Europe's most polluted countries and will need billions of euros to meet the European Union's environmental standards if it wants to join the bloc.

Rio Tinto has said it would adhere to all domestic and EU environmental standards at its lithium mine in Serbia.