Political opponents of Philippine President Rodrigo Duterte on Saturday turned a traditional celebration of a "People Power" uprising against dictatorship three decades ago into a protest march condemning his war on drugs.

The small political opposition and left-wing activists held separate commemorative events at the historic highway where more than a million Filipinos gathered in 1986, forcing Ferdinand Marcos and his family to flee to Hawaii after a 20-year rule.

Supporters of Duterte were due to hold a counter rally later on Saturday at a big Manila park, including a vigil backing the anti-drugs crackdown. Similar actions were planned in key cities across the country and in some capitals abroad.

The opposition warned of a possible return to authoritarian rule under Duterte, calling on Filipinos to stand up for truth and justice and demand a stop to the extrajudicial killings, which have claimed more than 7,700 lives in seven months.

"There is a president who is threatening to re-impose martial law and openly support the killings of thousand of people," staunch Duterte critic Senator Leila de Lima said in a message from her detention cell, a day after she was arrested for drug offences that she described as a vendetta. "The grim truth: in the last seven months under Duterte, there were more deaths compared to the 14 years of martial law under the Marcos regime."

Rights groups and a U.S. senator criticised Duterte for the arrest of de lima, describing it as "politically motivated" to silence his critic.

"President Duterte is effectively expanding his drug war from the urban poor to the legislative branch of government," Phelim Kine, deputy Asia director at Human Rights Watch, said in a statement. Thousands of left-wing activists marched on the main highway in Manila to demand "genuine change" promised by Duterte, calling for the resumption of peace talks with Maoist-led rebels and release of more than 400 political prisoners.

"This is particularly dangerous today when there are real efforts to rehabilitate the Marcoses and pave the way for their eventual return to Malacanang," Renato Reyes, Bayan secretary-general, said in a statement, referring to the presidential palace.

Former President Fidel Ramos and former senator Juan Ponce Enrile, hailed as the heroes of the 1986 revolt against Marcos, attended a mass at the main army base in Manila to mark the anniversary, which Duterte skipped.

Interior Minister Ismael Sueno had asked local officials to encourage supporters to join candlelight events organised by a pro-administration political movement.

Some cabinet members feared anti-Duterte protests might be used as a staging point to call for the president's ouster, although top security officials have made firm assurance about the loyalty of the army and police.