Philippine boxing icon-turned-senator Manny Pacquiao used the Bible to defend restoring the death penalty for drug traffickers as "approved by God", backing a controversial proposal of President Rodrigo Duterte.

The eight-division champion and devout evangelical Christian used his first senate speech to argue for capital punishment, calling it lawful and moral "especially in the eyes of God".

"When the government punishes, it's not an individual act. That's approved by God. That's what the Bible says", Pacquiao said.

"Having read the Bible on a regular basis, I am convinced that God is not just a God of mercy but he is also a God of justice."

Pacquiao was elected to the senate in May on the back of his popularity as a sports star among Filipinos, who consider him a national hero.

He and Duterte are political allies and both hail from the southern Philippines.

Duterte has declared a bloody crackdown on crime, especially illegal drugs, telling police to shoot drug suspects.

Since Duterte assumed office on June 30, police have reported killing over 400 drug suspects while the country's largest broadcaster ABS-CBN put the death toll at 852 to include reported summary executions.

The Philippines abolished the death penalty in 2006 following staunch opposition from the Catholic Church, the religion of 80 percent of Filipinos.

But Duterte has vowed to introduce executions by hanging for "retribution".

Pacquiao said on Monday he also preferred death by hanging or firing squad to reduce drug-related crimes.

Asked by a colleague if this was because hanging was cheaper than lethal injection, Pacquiao said in jest: "You'll just kick the chair. We can also say the chair was just knocked down."

Pacquiao said he filed several bills on the death penalty including for drug crimes, rape and murder because Philippine laws "lacked teeth".

But he did not answer questions on whether there were any studies showing that the death penalty led to lower crime rates.

In his 20-minute speech, Pacquiao repeatedly quoted Bible passages, prompting another colleague to refer to him as a "pastor".

He said he saw no contradiction between his beliefs and his support for the death penalty.

Addressing critics of the proposal, Pacquiao said: "God allows the death penalty to discipline the people and to punish those wrongdoers."