Calculate Current Inside the Resistor
In order to measure current, the meter should be connected in series with the circuit.
You are very fortunate not to have blown the fuse in your meter or completely destroyed the LED. 600mA is probably 20x the rated current of the LED, so it's quite possibly damaged.
The current is being limited by the resistance inside the batteries, the LED and (to a small extent) by the meter resistance.
This is a mistake almost everyone makes once. It's good practice to move the test leads back to the &quot;voltage&quot; terminals and switch the range to a high voltage range (AC or DC) before putting it away, so you don't forget next time.