In the capital Belgrade, people gathered on a major highway and a bridge and shouted slogans against the government.
Demonstrations were also held in other parts of the country.
- They allow foreign companies to do what they want in our country.
They set the table for us where everyone can come and take what they want, says the protester Vladislava Cvoric.
Serbian tennis star Novak Djokovic shares a photo of the protest on Instagram, and says that clean air, water and food are the key to well-being.
Large amounts of lithium have been found around the city of Loznica.
The mining giant has bought up land in the area, but has not yet received approval to start mining operations.
Lithium is used in electric car batteries, mobile phones and computers and the demand for the metal is high.
But extraction is often environmentally problematic.
Mining usually leaves large wounds in the ground, requires a lot of water and leads to extensive carbon dioxide emissions.
Rio Tinto plans to invest $ 2.4 billion in the project.
Critics accuse the government of trying to make illegal appropriations possible and of ignoring environmental consequences.
Last week, similar demonstrations were held.
One was attacked by masked men in Sabac, which led to anger on social media and accusations that the government is using hooligans to quell the protests.