Cassiobury Park has been converted into a graveyard as filming got underway on a mysterious production. 

Residents spotted 25 gravestones in a cordoned-off section of the Watford green space on Saturday (September 28), while over a dozen caravans and support vehicles appeared in the car park.

It is not clear what the crew was filming, although there are suggestions it was for an American horror film.

Update: A-list stars snapped filming in Cassiobury Park

Dozens of gravestones appeared over the weekend.Dozens of gravestones appeared over the weekend. (Image: Supplied)

Two locals were reportedly told by the site staff it was Warner Bros. filming for the fourth film in The Conjuring franchise "The Conjuring: Last Rites", which is due to be released next year.

Another claimed they had been told it was the third instalment in the 28 Days Later zombie film series "28 Years Later", which is also set for a 2025 release.

Watford Borough Council confirmed that "Mammoth Productions" is behind the filming, which is set to run until 7pm tonight, although the set was constructed over the weekend.