You can help by listening patiently. Do not be judgemental and let them know you believe them. Acknowledge how they are feeling. Do not dismiss their love for the abuser - we all engage in relationships hoping for the best.
Understand that your friend or family member might not be in a position to leave their partner yet. Be supportive, despite the fact you may disagree with this decision.
You can speak to the police by calling 101, alternatively speak to your local officers in the area you live - you can find them here, or if you'd rather you can e-mail us; firstname.lastname@example.org. However, remember in an emergency, where the incident is going or life is in danger, always call 999.