SINGAPORE'S first native-born head of government, former chief minister David Marshall, made a stinging attack on a bill passed in Parliament which presumes trafficking for possession of more than 30 grams of a controlled drug - bringing a mandatory death sentence. Mr Marshall, 85, who is resuming his law career after 15 years as Singapore's ambassador to France, described the law passed this week as ''radical and dangerous'' and said it eroded the normal protection of the individual. In a letter to The Straits Times, he said the principle of law in Singapore was that a person was presumed to be innocent unless he was proved guilty. ''What the present law seeks to do is to change that basic principle of law to one presuming a person is guilty and should be hanged unless he can prove he is not guilty,'' Mr Marshall said. ''If the legislature wishes to make possession a capital offence, it should have the courage to say so without going through the circuitous process of creating presumptions. ''Are long-term prison sentences not adequate? Do we need human sacrifice?'' Mr Marshall, a specialist criminal lawyer before he entered politics, said that he would like to argue a case before the Chief Justice.