Nine wooden stakes topped with pieces of a dead pig have been found on a plot of land where a mosque is scheduled to be built in Germany, according to police. It follows numerous protests against the proposed place of worship.

The stakes, which measured 1.5 meters (5 feet) tall, were planted on the plot of land in the east German city of Erfurt, DPA news agency said, citing local law enforcement.

Half a pig's head, pig feet, and pig bowels were placed on top of the stakes, at the site where the Ahmadiyya mosque is scheduled to be built.

Grässlicher Anschlag: Moschee-Gegner haben auf dem Baugelände der geplanten Moschee in Erfurt Schweineköpf auf Holzpflöcke gespießt. pic.twitter.com/IERNbvFvTn — Suleman Malik (@sulemannmalik) May 29, 2017

Erfurt police said they are unaware who planted the stakes and the accompanying pig parts, adding that an investigation into the case has been launched.

There have been numerous rallies against the Ahmadiyya mosque, one of which included protesters erecting large Christian crosses on a neighboring piece of land.

The Ahmadiyya community is a Muslim movement founded in the late 19th century in India. It has the same legal status as the Christian church in Germany, DPA reports.

Pigs are considered unclean by Muslims, and have been used in various anti-Islam protests in Germany and elsewhere.

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Earlier in May, a pig's head bearing insults against German Chancellor Angela Merkel was left outside her office. Merkel has faced stiff opposition to her liberal immigration policies which allowed over 1.1 million, mainly Muslim refugees into the country last year. However, it remains unclear if the attack was in response to her policy on asylum seekers.

Last month, severed pig heads were found hanging on a fence outside the Moroccan ambassador's residence in a Paris suburb.

In August 2016, a Hungarian MEP faced a barrage of criticism and bitter comments online after he suggested that severed pigs' heads would be a great deterrent against refugees trying to get into the country.

The placement of pig's heads and other parts of the animal’s carcass is considered highly offensive for Muslims, who abstain from consuming pork because the pig is considered an “impure” animal in Islam.