The children were rescued during raids in Nabi Karim area led by Sub-Divisional Magistrate Mani Bhushan Malhotra.
Vehicles of the raiding party including police and activists of the NGO Bachpan Bachao Andolan (BBA) were also pelted with stones by some unidentified people.
The raid was conducted after the NGO informed authorities that around 200 children were working in factories located in Nabi Karim, the BBA said in a statement.
"This is the fifth such raid by us in the area and just last month we had rescued 80 children from there," said Rakesh Senger of Bachpan Bachao Andolan (BBA).
"After rescuing 19 kids, we met with resistance as our vehicles were pelted with stones," claimed Senger.
The SDM said police was identifying the people who had attacked the civil society workers and as soon as they are identified, FIRs would be lodged against them all.
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"We have already arrested 4 employers and lodged FIRs against them," Malhotra said.
All the children rescued belong to Bihar and Uttar Pradesh.
The children were working for a weekly pay of Rs 30 for working upto 16 hours daily. All the children were below 14 years of age, the youngest one being seven years old.
The seven-year-old victim said he makes bags from 7 am in the morning till midnight. "I get Rs 20 every day with which I buy myself breakfast," said the boy, who was trafficked a year ago and sold to the factory by a local agent from his village.
Another victim, who is 11-year-old, said, "I was brought to Delhi for site seeing but on reaching, my brother put me in employment here. My employer pays me Rs 10 sometimes."