The arrest of Senator Leila de Lima shows that President Rodrigo Duterte is willing to use government to wage a personal vendetta, a human rights watchdog said on Friday.

Besides Congress, other “pillars of Philippine democracy” including the media and the judiciary, “should be worried about the future” because of the lengths Duterte is willing to go in order to wage his war on illegal drugs, said Phelim Kine, deputy Asia director of Human Rights Watch in a statement on Friday.

“By arresting Senator Leila de Lima on politically motivated drug charges, President Duterte has effectively expanded his 'drug war' from the urban poor to the legislative branch of the government,” he said.

Earlier this week, Human Rights Watch urged the government to drop the "politically motivated" drug charges against De Lima, and accused the administration of abusing the judicial system to punish critics of its anti-drug campaign.

De Lima has been one of the administration's most vocal critics and initiated a Senate inquiry into alleged drug-related extrajudicial killings.

Human Rights Watch also slammed the administration's alleged disregard for human rights, as shown in the way it waged its anti-drug war, which has resulted in thousands of deaths.

“It’s more important than ever that concerned lawmakers and foreign governments step up to denounce the Duterte administration’s disregard for basic human rights," Kine said.

The government has repeatedly denied that cases against De Lima were politically motivated.