Yes, you are eligible for civil service exam with lower than 50%, the basic requirement is that the applicant should have completed graduation. Civil Services Examination is conducted by the UPSC (union public service commission).

Following are the details of this examination:


Nationality:
A candidate must be a citizen of:
A citizen of India, or
A subject of Nepal, or
A subject of Bhutan, or
A Tibetan refugee who came over to India before 1st January, 1962 with the intention of permanently settling in India, or
a person of Indian origin who has migrated from Pakistan, Sri Lanka, Burma, East African countries of Kenya, Uganda, the United Republic of Tanzania, Zambia, Malawi, Zaire, Ethiopia and Vietnam with the intention of permanently settling in India.


Required qualification:
The candidate must hold a degree of any of Universities incorporated by an Act of the Central or State Legislature in India


Age Limits
Candidate must have attained the age of 21 years and must not have attained the age of 30 years on 1st August