No, you cannot do it on a live filesystem. I cross my fingers that using fdisk on a live partition did not completely damage it. Always perform such operations from the system booted from LiveCD / rescue CD / other partition. Partition may not be writeable during that operation.
Do not jump to my instructions immediately; maybe someone will come up with a better plan. I include these instructions just in case no one else responds.
Start the system using LiveCD and go to rescue mode. Resize the partition as planned using resize2fs. Try to mount the resized partition and see whether it works. If you have anything valuable (i.e., other than OS / software), back it up immediately. If everything went OK, try to reboot.