Since on our keyboard SysRQ is triggered by Fn alt key we should press

ALT+Fn+SysRQ+<letter>

These key combinations are very useful when Xorg crashes since the kernel will respond regardless of whatever else it is doing, unless it is completely locked up. With these keys combos you can try to regain control over your system:

ALT+Fn+SysRQ+<R> Turning off keyboard raw mode

ALT+Fn+SysRQ+<S> Syncing the disks and filesystems (to minimize data loss)

from this sequence you can use ALT+<F1-F6> in order to switch to a tty terminal console, where you can log in and actually see which process is responsible of the system lock via top command as well as do whatever you wish.

If you manage to fix the problem (or simply wanted to see what SysRQ was like) and want to get back to the graphical user interface you have to go to tty7 via ALT+F7. In case X was the responsible and you had killed it you can start it again via startx.

ALT+Fn+SysRQ+<E> SIGTERM, last chance for processes to end

ALT+Fn+SysRQ+<I> SIGKILL, all processes termination

ALT+Fn+SysRQ+<U> remount disks and filesystems in read-only mode

ALT+Fn+SysRQ+<B> reboots

You can find a comprehensive explanation of SysRQ key and keys combos:

http://lxr.linux.no/linux/Documentation/sysrq.txt

You can check if Sysrq keys are allowed with ‘cat /proc/sys/kernel/sysrq’

if ‘1′ is showed all key combo are allowed (this is default).

http://en.wikipedia.org/wiki/Magic_SysRq_key

Thank you MaNRo for assisting me!