Clear some space in the middle of the room and put three chairs down. Group is seated round the edge. Invite three students to come and sit on the chairs, and give them a question to discuss for about ninety seconds. The best questions are those where people might have different views, eg. 'Is the Enlightenment relevant today?', 'Is the world safer with only one superpower?', and so on. Reassure them that they will only have to talk for ninety seconds - not a second longer.
Tell the rest of the class to make notes on what's said. Hopefully they should all be familiar with the topic (Enlightenment literature, art, etc; world politics after the fall of the USSR, and so on). The key is to give the rest of the class something to do, and to reassure the three students sat in the middle of the room that their peers are busy with their notes and not staring at them intently.
Start the students talking. Sit back and don't intervene, even when awkward silences occur. On the stroke of ninety seconds, place a fourth chair in the centre of the room. Pause the conversation and let everyone know that if they have something to contribute then they can sit on the empty chair and join in the discussion, and that one of the original discussants must then leave to ensure that there is always an empty chair for anyone who wishes to join in. Generally the more controversial the topic the more likely you are to have people willing to sit in the empty chair, and this calls for a good judgment about levels of knowledge and aligning the question with what your students are likely to know.
You can let the discussion roll on, or cut it short and give the students a new topic and repeat the process.