I've been there, and so will some people reading this. If your body language is aggressive or defensive and you display anxiety or anger, none of these things work in your favour.
You must relax and make a fresh start. Pretend you are teaching them for the first time. Eye contact is crucial; stand at the door and greet each as they enter. Repeat this at the end of the lesson.
It is very important that children think you like them whatever their behaviour. I recently attended an excellent course on classroom management, given by the behaviour management expert Andy Vass, and learnt many things well worth sharing.
Here is an interesting point he made: if you go into a classroom looking for bad behaviour you will find it - but in the meantime, you may also miss the good behaviour. Start positively, look for good behaviour and reward it with positive comments. Equally important, be sure to establish rules concerning poor or inappropriate behaviour.
Andy Vass also emphasised that one should never give a sanction without first offering a choice. Giving the student the option of either following the rules or choosing to be sanctioned gives him or her a way to determine the outcome. This is subtly empowering, and is more likely to get a result than merely ordering a student to sit down or to stop talking, which all too often leads to confrontation.
I am not saying the effect will be instant, but if you are consistent you should establish a teaching environment in which all parties understand their parts, and sanctions and rewards are clear.