Neighbours in Kingsbury gathered to stop workmen who parked up in the area to erect a 5G telecoms mast despite the application being refused.
An application from Hutchinson UK Ltd to build an 18m mast for Three network on the green space in Tewkesbury Gardens was refused by Brent Council's planning department in June.
However, on October 25, workmen appeared with a skip and bags of cement claiming, according to residents, they had approval.
Neighbours gathered to stop the work and, after flagging the incident, were told by the council that permission had not been granted.
But a spokesperson for Three claims the refusal was invalid as it was given more than 56 days after the application.
Kingsbury resident Simran Vagadi said: "The greenery is for our kids to play on, not for a mobile provider to build a tower in the middle of it.
"It's an eyesore for the whole neighbourhood. They lost the application [but] now they are trying to build it [anyway]."
Sanjay Trivedi, who lives opposite the site, said: "Whoever sent these workmen out has done so in breach of the planning decision.
"It seems to be the case that residents are left on their own. This is shameful, to try and destroy such a beautiful public open space."
Neighbours objecting on the council's website also cited health reasons when objecting back in May. They were fearing a greater amount of electromagnetic radiation emitted by 5G masts.
In refusing, Brent Council wrote: "While it is noted that the proposed equipment would add to the future telecoms network infrastructure, the siting and appearance of the equipment would have a detrimental impact on the setting of the grass open space and character of the area."
A council spokesperson added: "The proposed works do not have planning consent and as soon as we were informed that contractors had arrived we instructed them to suspend work and leave.
"An enforcement notice was subsequently served and all materials and machinery removed."
A Three spokesperson said: "We have since stopped work but submitted a subsequent appeal.
"5G rollout is vital for residents and businesses in Kingsbury.
"We want to offer the community a reliable network experience and this site will be critical to making that happen.”
Brent Council had not responded to the claim it served an invalid notice as the paper went to press.
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