Retrieving svg element inside a script

Hi,
I need to retrieve an SVG element using a ecmascript event.
Let's say my SVG code has a circle element with the id "circle1" and 
another element that responds to an onclick event and runs a ecmascript 
function. How do I retrieve my circle in this function. I thought the 
following line could work but it's not.
var circle = document.all("circle1",0);

Any idea ?

Thanks