Ban Ki-Moon, the UN secretary-general, said earlier this week that "conditions do not exist at the moment for free and fair elections in Zimbabwe", and that "there has been too much violence, too much intimidation" for the vote to take place.
"A vote held in these conditions would lack all legitimacy," he said.
Zimbabwe's poll was reduced to a one-horse race earlier in the week when Morgan Tsvangirai, leader of opposition party Movement for Democratic Change (MDC), withdrew complaining of intimidation and violence by supporters of Mugabe's Zanu-PF party.
The MDC says that at least 80 of its supporters have been killed by Zanu-PF loyalists and another 200,000 people have been displaced in what it calls Mugabe's "campaign of intimidation" to deter people from voting.
Al Jazeera's Haru Mutasa, reporting from the capital, Harare, said activity at polling stations was slow on Friday and people were being marched down to polling stations, apartment by apartment.
"One is saying, oh for goodness sake Mr Mugabe, you can end this tragedy, step down"
South African Nobel Peace laureate
Speaking at a G8 meeting in Kyoto, Japan, Condoleezza Rice, US secretary of state, said Washington would consult other members of the UN security council on the next steps to be taken regarding Zimbabwe.
"This kind of sham cannot possibly produce a legitimate outcome," Rice said.
Further sanctions against Zimbabwe would be the specific topic of discussion by the UN members, Frank-Walter Steinmeier, Germany's foreign minister, said.
Italy on Friday joined its voice to Western-led condemnation of Zimbabwe's presidential poll, calling on the EU to discussing withdrawing its ambassadors from Harare.
Franco Frattini, Italy's foreign minister, said he asked Bernard Kouchner, his French counterpart, to start that process, given that France's six-month European Union presidency starts in July.
"I will ask Kouchner ... to start a rapid consultation process for the recall of the ambassadors from Zimbabwe," he said in Kyoto.
In Brussels, a European Commission spokeswoman said on Friday the election was "hollow and its result will be equally hollow and meaningless".
Desmond Tutu, South Africa's nobel peace laureate, called on international leaders to declare Zimbabwe's election "illegitimate" if Mugabe claims that he is the duly elected president of Zimbabwe.
"It is that we have seen unfold before our very eyes, a sad tragedy, a country that use to be a bread basket turning into a wilderness with shops that are standing empty, with inflation that I mean is almost difficult to comprehend and one is saying, oh for goodness sake Mr Mugabe, you can end this tragedy, step down," Tutu said.
Tom Casey, the US state department spokesman, said the election was a "sham process run by a government that will have no legitimacy, no matter what it says".
Casey called on heads of state from the AU, who are due to meet next week in Sharm el-Sheikh, Egypt, to act.
|Tsvangirai pulled out of the presidential run-off citing attacks on his supporters [AFP]
"Certainly we would hope that they would continue to speak out in opposition to this completely fraudulent electoral process that is now underway and put their weight behind international efforts to reach some kind of political solution."
The AU said on Friday it was convinced it could "sort out" the election crisis.
"I am convinced it will be solved in a credible way. But please give us time to solve it with our heads of state," Jean Ping, chairman of the AU Commission said on Friday.
Neighbouring Zambia also joined international criticism of Zimbabwe's conduct during the run-off poll.
Mike Mulongoti, Zambia's minister for information, said: "When we tell our neighbours that we'll help them fight for independence, we were not agreeing to bringing a black opporessor after removing a white oppressor.
"If we knew this would be the case, we would have not appeared in the liberation wars," he said.
Amnesty International (AI) attacked what it said was the atmosphere of violence surrounding Zimbabwe's elections on Friday while criticising the AU's "deafening" silence on the situation.
AI said elections were taking place "against a backdrop of widespread killings, torture and assault of perceived opposition supporters".
It added that it was "deeply disturbed by the continuing campaign of state violence and intimidation as part of a deliberate strategy by the Zimbabwean government to ensure that Robert Mugabe wins today's presidential election".