triggers, transactions and locks

Dear All,

Is there a way to use locks within a trigger? My example below gives the error:
ERROR: unexpected error -8 in EXECUTE of query "BEGIN"
CONTEXT: PL/pgSQL function "insert_into_t1" line 6 at execute statement

Thanks

Colin

CREATE FUNCTION insert_into_t1() RETURNS trigger AS'
DECLARE
set_qry text;
BEGIN

set_qry:=''BEGIN WORK'';
execute set_qry;
LOCK t1;
INSERT INTO t2 VALUES (11);
COMMIT;
RETURN NEW;
END;
' LANGUAGE 'plpgsql' SECURITY DEFINER;

CREATE TRIGGER insert_into_t1 BEFORE INSERT
ON t1 FOR EACH ROW EXECUTE
PROCEDURE insert_into_t1();