The Chinese government has created a "climate of fear" in Hong Kong designed to skew the result of this Sunday's election, a human rights group says.
Pro-democracy candidates are hoping to win the people's vote
According to Human Rights Watch, an intimidation campaign is under way to undermine the pro-democracy opposition.
The Hong Kong government has reacted angrily, claiming the report paints a distorted picture.
Sunday's vote will elect politicians to the territory's mini-parliament, the Legislative Council (Legco).
Human Rights Watch said that there was a "toxic political climate" in the territory, unparalleled since sovereignty of Hong Kong was handed from Britain to China in 1997.
Activists say China is breaking a promise it made to respect Hong Kong's autonomy and Western-style civil liberties.
Specific incidents Human Rights Watch alleges in its report include the Chinese government, or people acting on its behalf, have sent threatening letters and phone calls, and carried out vandalism and arson against pro-democracy targets.
In one case, a businessman was told to take a photograph of his completed election ballot or his business would suffer, the group claimed.
"The past 12 months have seen some of the most worrying violations of human rights since the 1997 handover," the report
HONG KONG VOTE
All 60 seats in Legislative Council up for grabs
Only half are decided by direct elections
Other half reserved for business and professional groups
Opposition candidates allege that a recent tour of China's Olympic medal winners in Hong Kong was designed to boost support for pro-Beijing candidates.
The triumphant Olympic team was welcomed in the territory earlier this week against a background of Chinese flags and national songs.
Human Rights Watch admits that the direct intervention of Beijing officials is virtually impossible to prove.
The government can therefore insist, as it has done for months, that it has done all it can to investigate claims of electoral fraud and intimidation, says the BBC's Hong Kong correspondent Chris Hogg.
The people of Hong Kong go to the polls on Sunday, to elect their Legislative Council, which does not set policy but can vote on legislation.
Half of the 60 seats will be decided by Hong Kong's 3.2 million registered voters.
The other 30 are picked by largely pro-Beijing professional groups.