Pakatan Rakyat has demanded a probe into the mysterious death of a political aide, who apparently plunged from the 14th floor of anti-corruption offices where he was being questioned.

Opposition figures expressed disbelief over the death of Teo Beng Hock, 30, an assistant to a member of the state cabinet in Selangor, which is ruled by the Pakatan Rakyat opposition alliance.

Pakatan leader Anwar Ibrahim called for a royal commission to probe the incident, which he said was part of a pattern of government harassment.

"We don't see this as an isolated incident. This is a systematic attack on the Pakatan Rakyat," he told a crowd of several hundred who gathered outside the building where Teo died.