The ILP is mandatory to enter and stay in the tribal dominated mountainous state of Mizoram which shares borders with Myanmar and Bangladesh.
"Police with the help of local NGOs launched a drive across the state since Friday and arrested 503 non-tribals living in Mizoram without valid ILP. These people belong to various northeastern states including Assam," Aizawl district police chief L.R. Dingliana Sailo told reporters.
He said: "All 503 people were pushed back to neighbouring Assam after they were found guilty by a local court in violation of the ILP regulations."
People from other states, especially those who are non-tribals, have to obtain an ILP before entering Mizoram. The validity of the ILP could be extended.
The non-residents can also procure temporary ILP at the airport, Mizoram government offices in Delhi, Kolkata, Assam, police stations in border areas and other areas.
The ILP is also in force in two other northeastern states - Nagaland and Arunachal Pradesh - to protect the indigenous people from becoming a minority.
Several parties and NGOs have often organised protests to have the ILP promulgated in Manipur and Meghalaya also.
The demand was earlier rejected by the home ministry saying the ILP would not be extended to any other state in India as this is "unconstitutional" since it is in violation of Article 19 of the Indian constitution.