Tens of thousands of people took to the streets of Munich on Thursday to protest planned changes to police powers in the prosperous southern German state of Bavaria.

Police estimated that around 30,000 protesters turned out on a public holiday to oppose a law approved by state parliamentarians last summer that massively widens the scope of police powers.

"We're demonstrating loudly but peacefully," said Simon Strohmenger of noPAG, the umbrella organization that staged the demonstration against the law.

Critics lament that the law, which has not yet gone into effect, allows judges greater discretion in detaining suspects, increasing the preventative detention period from 14 days to 3 months.

The law also enables police to monitor suspects' phones and computers on a preventative basis.

Backlash against the law has already led to modifications: Facial recognition technology will no longer be used in video surveillance.