Some assessment should be done before answering this, but to get some relief from the muscular spasm the following things may help:
1) Rest - absolutely important
2) Anti inflammatory and muscular relaxant medications
3)Avoiding any physical activity that involves bending and lifting.
Next Steps
Consider consulting an orthopaedic surgeon for acute back pain which is going down your leg, could be sciatica or a vertebral disc issue.
Health Tips
Do not lift heavy, or bend forward.