The first U.N. special session to address global drug policy in nearly 20 years bristled with tension Tuesday over the use of the death penalty for drug-related offences, as countries wrestled over whether to emphasize criminalization and punishment or health and human rights.

The outcome document adopted by member states included no criticism of the death penalty, saying only that countries should ensure that punishments are "proportionate" with the crimes.

"Disproportional penalties ... create vicious cycles of marginalization and further crime," Mexican President Enrique Pena Nieto told the gathering. He also called for the decriminalization of marijuana for medical and scientific purposes and said the international community's responses to drug issues is "frankly, insufficient."

He said Mexico in the coming days would announce specific drug policies with an emphasis on health and human rights.

At least 685 people around the world were executed on drug-related offences in 2015, said Chiara Sangiorgio, a death penalty expert with Amnesty International. The rights group says 30 countries have laws that punish drug-related offences with the death penalty.

Indonesia, which last year executed 14 people, mostly foreigners, convicted of drug-related crimes amid an international outcry, defended its stance Tuesday, saying the death penalty is not prohibited under international law.

China, which along with countries such as Saudi Arabia and Iran carries out executions for drug offences, signalled little flexibility on its approach.

"Any form of legalization of narcotics should be resolutely opposed," Public Security Minister Guo Shengkun told the gathering.

Prior to this week's three-day meeting, Democratic presidential candidates Hillary Clinton and Bernie Sanders, rock star Sting and hundreds of others sent an open to U.N. Secretary-General Ban Ki-Moon letter saying the war on drugs has failed. It said that for decades, governments have focused resources on repressing drug use, resulting in the imprisonment of millions of people, mostly the poor and ethnic minorities, and mostly for non-violent offences

The letter's signers, including former presidents of Mexico, Colombia, Brazil, Switzerland and others, joined a growing number of government officials and drug policy analysts calling for a shift in global drug policy from emphasizing criminalization to health and human rights.

Hundreds of government officials, representatives of non-governmental organizations and individuals from civil society are attending the General Assembly special session at U.N. headquarters.

The last special session on the topic in 1998 ended with the lofty but unattainable goal of ridding the planet of illegal drugs by 2008.

On the non-medical use of drugs, the outcome document adopted Tuesday says countries should "develop and implement countermeasures and supportive public health, education and socioeconomic strategies to effectively address and counter the non-medical use and misuse of pharmaceuticals that contain narcotic drugs and psychotropic substances, while ensuring their availability for legitimate purposes."

Last month, The Global Commission on Drug Policy -- whose members include former presidents of Mexico and Brazil, as well as former U.N. Secretary-General Kofi Annan and Virgin Group founder Richard Branson -- said in a statement that discussions drafting the outcome document adopted Tuesday relied too heavily on an outdated approach that emphasizes criminal justice and prohibition.

The group argued that the emphasis should instead be on alternative approaches including abolishing capital punishment for drug-related offences and a focus on treatment.