Kyiv, Ukraine - Ukrainian authorities reportedly thwarting a coup attempt on Sunday, when a group attempted to mount "provocations" in Kyiv as the country marked its Constitution Day.

The plans involved occupying parliament during a demonstration and setting up a "provisional government," Ukraine's SBU intelligence service said.

Four men were being investigated, with two held in custody, the report added. Weapons and ammunition had been found in raids.

The suspects, who authorities said were part of a group "known for its anti-Ukrainian actions since 2015," could face up to 10 years in prison if found guilty. The investigation is being led by the authorities in Ivano-Frankivsk in the west of the country.

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An SBU press release cited by the Kyiv Post said: "The perpetrators planned to spread information about the 'disturbances' in Kyiv through domestic and foreign information resources. In this way, they hoped to shake up the social and political situation within our country, which would work in favor of the Russian Federation."

A security zone has been declared around the government quarter in Kyiv, where the access and the right to demonstrate have been restricted.

In May, two officers in Ukrainian President Volodymyr Zelensky's protection unit were arrested for their alleged involvement in an assassination attempt.