He also called on the authorities to provide a safe and secure environment, which will allow Tanzanians to exercise their civil and political rights, the statement said.
An inclusive electoral process and a broad effective participation of political parties and their candidates, particularly women, remain essential for safeguarding the progress made by the east African nation in consolidating stability, democracy and sustainable development, the statement added.
The polls are to be held on Wednesday, 28 October.
At a regular media briefing, in Geneva, OHCHR spokesperson Ravina Shamdasani spoke of "worrying reports" of intimidation, harassment, arbitrary arrests and physical attacks against political opponents, journalists, women human rights defenders, and other activists.
"This repression of dissenting voices intensified in the lead up to the elections, when the rights to freedom of expression and political participation should be upheld, not repressed," she added.
The OHCHR spokesperson raised particular alarm over reports that three people were reportedly killed on Monday night and others injured on Pemba Island in the Zanzibar archipelago where police fired live ammunition in clashes with opposition supporters.
"We urge the authorities to ensure prompt, transparent, independent investigations into the incident and urge all actors to refrain from any acts of violence," she said.