Is it possible to only allow deletes from a table via referential integrity cascades?

I have two tables joined by a foreign key constraint:

> CREATE TABLE test_master(
> id SERIAL PRIMARY KEY,
> foo TEXT
> );
>
> CREATE TABLE test_detail(
> id SERIAL PRIMARY KEY,
> master BIGINT NOT NULL REFERENCES test_master(id) ON DELETE
> CASCADE ON UPDATE CASCADE,
> bar TEXT
> );

Is there a way to block deletes on the "test_detail" table that will
only allow rows to be deleted if it is the result of deleting the
corresponding "test_master" record? In other words, I'd like to
disallow direct DELETE commands like this:

> DELETE FROM test_detail WHERE id = 1;

while allowing a command like

> DELETE FROM test_master WHERE id = 1;

to subsequently delete via CASCADE all "test_detail" rows that
reference test_master # 1.

I've tried using rules and revoking privileges, but both of these
approaches fail when trying to delete from "test_master".

Thanks in advance,

Chris