Rio By Foot offers free walking tours in Rio de Janeiro and this is their starting point. You'll meet the guides right in the city center and explore the area on foot.

I have done this free walking tour in the past and had a really good time, even as a local. There were so many details and special info provided by the guides, that even I didn't know. I highly recommend it.

Exploring the city center on your own can be tricky. Joining a tour like this is a great alternative.