+1 for DBMS_SCHEDULER and DBMS_JOB approaches, but also consider whether you ought to be using a different approach.
If you have a procedure which executes in a row-by-row manner and you find that it is slow, the answer is probably not to run the procedure multiple times simltaneously but to ensure that a set-based aproach is used instead. At an extreme you can even then use parallel query and parallel DML to reduce the wall clock time of the process.
I mention this only because it is a very common fault.