Transaction within plpgsql

I am coding a procedure to purge data from a table. I want to commit the
data I'm purging at intervals (for example, every 1000 rows). The
problem I'm encountering is that when I have

begin transaction;
commit;

statements in my plpgsql function it doesn't compile.

How can you explicitly create transactions within a plpgsql function?

Thanks,

Jed.