Philippine President Rodrigo Duterte's campaign to bring back the death penalty for drug-related crimes has cleared a major hurdle, with supporters backing it in congress but critics denouncing the plan as "inhumane".

The death penalty bill - along with a proposed measure to punish children as young as nine as adult criminals - are key planks of Duterte's controversial drug war that has already claimed more than 7,000 lives.

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A majority of politicians in the lower house of congress passed a second reading of the bill on Wednesday night, clearing one of the biggest obstacles in proponents' plans to make the death penalty legal by May.

A third and final reading still needs to be held next week, although with no more debates both sides agree passage is a formality. Then the Senate, which is similarly dominated by Duterte's allies, would pass a counterpart bill.

"We have hurdled the most difficult part," congressman Reynaldo Umali, a sponsor of the bill, told AFP news agency.

Opponents voiced anger about the Philippines' attempt to bring back the death penalty, 11 years after it was revoked, highlighting among many concerns a corrupt justice system that would lead to innocent people being executed.

"The decision is inhumane, shameful and blatantly disrespectful," Father Jerome Secillano, executive secretary for public affairs at the Catholic Bishops' Conference of the Philippines, said in a statement.

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"Let me reiterate this, criminals should be punished and victims should be aided, but the punishment should not be death. Due to our flawed and dysfunctional criminal justice system, there is a great chance that innocent people may become victims of wrongful convictions."

The Catholic Church, which counts 80 percent of Filipinos as followers, had led the opposition to abolish the death penalty in 2006.

A Filipino holds a placard during a public forum dubbed 'Rise Up!' - a campaign to document cases of drug-related killings [EPA]

Secillano and opposition lawmakers also criticised tactics used to ensure the bill was passed, such as the curtailing of debates and only allowing a vote by voice so lawmakers would not be specifically identified as having supported it.

The speaker of the house also threatened to strip lawmakers of committee leadership positions if they voted against the bill.

"This is a chamber of puppets and bullies," congressman Edcel Lagman, a longtime opponent of capital punishment, said after his efforts to block the bill were voted down.

Just drug crimes

The bill limits the death penalty to drug-related offences only.

Possessing 500 grammes of marijuana, or 10 grammes of cocaine, heroin or ecstasy, would be crimes punishable by execution, as would manufacturing and selling drugs.

People who commit serious crimes such as murder and rape while under the influence of drugs could also be executed.

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The bill allows for execution by hanging, firing squad or lethal injection.

Duterte won the presidential election last year after pledging an unprecedented campaign to eradicate illegal drugs.

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Since he took office in May, police have reported killing thousands of people in the drug crackdown, claiming all deaths were in self defence. Thousands of others have died in unexplained circumstances.

Rights groups and other critics say Duterte is presiding over widespread human rights violations, with Amnesty International warning the killings could amount to a crime against humanity.

Many Filipinos support Duterte and his drug war, arguing extreme measures must be taken to halt crime.

The United Nations had warned bringing back the death penalty would violate international conventions the Philippines had already ratified.

After capital punishment, another priority bill for Duterte is lowering the age of criminal liability to as low as nine-years old, from 15 currently.

Source: News agencies