The incident was reported just after 5 p.m. at Rudd playground at Bushwick Avenue and Aberdeen Street.
Police say an 11-year-old boy entered the fenced-off area and ran across the grates -- but he didn't know a grate was missing.
He fell 25 feet and landed adjacent to the L train tracks.
"He didn't realize that there was an open hole so when he ran he just fell," eyewitness Shania Crayton said.
Witnesses said the boy's pregnant mother tried to jump down, but instead, half a dozen men playing basketball nearby along with the boy's father sprung into action.
They worked together and even used a dog leash to pull him to safety.
He was taken to Kings County Hospital but his exact injuries were not yet known.
The NYC Parks Department released a statement saying, "The health and safety of this child is on hearts and minds. We are happy to hear that he was safely removed from inside the subway grate that runs through Rudd Playground by good Samaritans."
They said the playground was recently refurbished and the subway grate was completely fenced off.
It is unclear how the child was able to get in and fall through, but officials suspect the fence was vandalized and it had been peeled back.
The MTA boarded up the hole and released the following statement:
"Our thoughts are with the child and his family. We have dispatched a team to investigate what occurred and ensure the site is safe and secure. The safety of New Yorkers is always our top priority."
Parents say the fencing alone is not enough.
"For a train going underneath it, like that's the protection for all these kids that are here, it's just not enough," a witness named Parker said.
Parents told Eyewitness News they called 311 to report the missing grate, fearing something like this would happen.
The investigation into the incident is ongoing.
MORE NEWS: Video shows NYPD transit officers rescue man who collapsed onto subway tracks
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