Metro Manila (CNN Philippines, July 31) — Twenty workers at NutriAsia and supporters calling for their regularization were arrested by members of Meycauayan, Bulacan police on Monday, following a violent dispersal.

They were arrested for direct assault, carrying illegal firearms, and illegal drug possession after one person was allegedly found to carry a .22 caliber revolver and four sachets of suspected shabu.

Two hundred workers from a contracting agency of food condiment giant NutriAsia were holding a strike outside the Marilao, Bulacan plant when rocks started flying in the air, leaving several protesters and their supporters — including an elderly lady — bloodied.

Protesters claimed the police initiated the violence to disperse the picket line, but the authorities and the company alleged the rallyists fired a warning gunshot and started to hurl rocks at police and guards.

The Philippine National Police (PNP) said it has directed its Regional Internal Affairs Service to investigate the incident and "recommend necessary corrective measures and sanctions against our police personnel if so warranted."

"As a general policy, the involvement of PNP personnel during strikes, lockouts and labor disputes in general shall be limited to the maintenance of peace and order, enforcement of laws, and implementation of legal orders of the duly constituted authorities," the national police said.

It also called on both sides, the labor organizations and the company "to find better ways of settling labor disputes."

NutriAsia, meanwhile, said an elderly woman was hurt by the rock hurled at her, which the company claims came from the protesters.

The violent dispersal comes weeks after the Labor Department ordered NutriAsia to regularize 80 workers of one of its the agencies. It said the food condiment company is engaged in the illegal practice of labor-only contracting with one of its contractors.

Related: DOLE orders NutriAsia to regularize 80 workers of its contracting agency

Kilusang Mayo Uno and Kadamay slammed the police for planting fake evidence on the arrested protesters.

"Once again, it becomes clear what role drugs have in the Duterte administration. It is a tool to wage a murderous war against the poor as well as a justification for the police to raise the level of repression directed against activists and human rights defenders," Kadamay said in a statement.

Among those arrested were journalists from progressive media outfit Altermidya and a member of a University of the Philippines-Diliman publication who were covering the NutriAsia picket line.

The National Union of Journalists of the Philippines condemned the arrests.

"The assault, threats and arrests of our colleagues is a clear attack on press freedom and highlights the increasing dangers journalists face in these increasingly troubled times… We demand just as strongly that they forget the ludicrous notion of filing trumped up criminal charges against our colleagues," NUJP said in a statement.

In a statement sent on Tuesday, NutriAsia said it has no plans to pursue charges against the arrested students, but did not discuss the charges against the others who were arrested.

This is the second violent dispersal of NutriAsia workers. On June 14, violence erupted at the NutriAsia plant when police attempted to break up the picket line. The workers were on strike for 12 days to protest alleged contractualization, low wages, and unhealthy working conditions.

Related: Police slammed for 'violent dispersal' of workers in NutriAsia

NutriAsia: Workers initiated violence

NutriAsia alleged the protesters — composed of 200 workers of contracting agency Toll Packer BMirk and their supporters — fired a gunshot and started throwing rocks at their guards and the police.

"It is unfortunate that the strikers staged a violent attack while a DOLE-led mediation meeting among stakeholders, including the union organizer, was at the same time ongoing with the objective of seeking a peaceful settlement for the strikers," it said in a statement on Monday.

It added the striking workers used children and elderly to protect their picket.

"It is even more disturbing that the strikers had used young children and elderlies as frontliners and human shields as they pushed their way to barricade the plant entryways. These children and elderlies clearly have nothing to do with the labor issue at hand," it said.

NutriAsia is one of the largest producers of condiments in the country, which include Datu Puti, Mang Tomas, UFC, Papa, and Jufran.