Brazilian protests: why do the police use shields in protests?

Protest in Rio de Janeiro June, 17, 2013 in front of legislative assembly

The protests, triggered by a hike in mass transit fares that began more than a week ago, is spreading across the country raising many other problems.
The protesters don't want political parties but it is inevitable that they claim that they are supporting and even among the crowds without any prove.

Tear gas and rubber bullets are being used but it seems that there is not a single pacific demonstration in the world without the presence of the police armed as if combating armed people. Why they use shields is a mystery  since all the violence are created by their forces.

Maybe they fear the flowers.