This is a good question, as it takes a lot of sensitivity. In light of this, let me offer a few suggestions.
First, if you know the people of your team and what they are like, then you need to use this to your advantage. Announcing that you are a team leader can be tricky, as it might sound like self-promotion. If the people are the jealous type, then you need to think about strategies. For example, Nelson Mandela stated, "Lead from the back - and let other believe they are in front." Drew Faust of Harvard also used this tactic to lead the Harvard faculty.
Second, if the team is fine with your leading, then you might want to be straightforward and let the team know directly. Here is another option. If someone appointed you as a leader, then the person who appointed you might want to make the announcement for you.