Tested with Firebird 2.1.3 Classic (Windows)

set term !! ;
execute block (
par1 smallint = :par1)
as
begin
if (exists (select * from rdb$database where rdb$relation_id = :par1)) then
exit;
end !!
set term ; !!

/*******************************************************************************
The next statement causes the following error:

Invalid token.
Dynamic SQL Error.
SQL error code = -104.
Token unknown - line 2, column 16.
Is.
*******************************************************************************/