Implicit Lock Row

I currently develop an interface to simulate a indexed sequential file management with PostgreSql. I must reproduce the same philosophy used of control of locking of the records.

I seek a solution to lock and unlock implicitly a row of a table. The locking of several rows, of the same table or various tables, can last a long time and consequently locking cannot be included in a transaction for not to lock the whole table for the other users.

There is a viable solution with PostgreSql?

There is an accessible basic structure of locking?

Thank you.