Political repression and partisanship by the security forces in the Democratic Republic of Congo could lead to bloodshed in this month's presidential election, the United Nations Joint Human Rights Office said in a report on Wednesday.

Polls show President Joseph Kabila is likely to win re-election in the parliamentary and presidential vote on November 28, but his challengers have strong support among the country's many ethnic communities.

Congo's last election in 2006 capped a difficult peace process after two of Africa's bloodiest wars. There are still pockets of clashes in the east, and frustration across the impoverished country at the lack of development.

Continued repression and rights abuses may increase the likelihood of individuals and political parties resorting to violent means, endanger the democratic process and lead to post-electoral violence, the U.N. report said.

The report documented 188 cases of election-related violence between November 2010 and September 2011. At least four people have been killed during political demonstrations and more were injured when the police used tear gas and shot live ammunition in the air to break up crowds, it said.

Security agents responsible for the human rights violations in this report appeared at times to be overzealous, acting on behalf of local or provincial authorities, or both, it said.

A law banning insults against the head of state had been used to prosecute people who criticised Kabila, and police agents had beaten, threatened and arrested civilians just for wearing opposition t-shirts, it said.

In spite of constitutional guarantees, those seeking to express their opinions and their fundamental freedoms of assembly and association were often subjected to abuse by State agents and saw their right to physical integrity violated.

An opposition supporter has been in detention since March 18 for possessing a journal questioning Kabila's nationality.

In one incident, the report said 13 people were injured when riot police shot in the air to disperse a crowd that gathered to greet a Kabila rival in the eastern city of Goma last December.

The situation in the East of the country is of particular concern, as political parties have reportedly been targeted and many of their members have been deprived of their liberty or subjected to ill-treatment and threats, the report said.

Security forces and judicial officers need training and political parties should issue statements to promote peaceful participation, it said. They should also call upon their supporters, especially youth, to refrain from violence.

(Reporting by Tom Miles; Editing by Peter Graff)