देवेषु when devatas, विप्रस्थितेषु सत्सु after having departed, महामुनि: mighty ascetic, विश्वामित्र: Visvamitra, ऊर्ध्वबाहु: with arms raised upwards, निरालम्ब: without support, वायुभक्ष: subsisting on air, तपश्चरन् while performimg.
After the departure of the gods, mighty ascetic Viswamitra raised his arms up, stood without support, subsisting on air, continuing with his austerities.