The weekend elections and referendum in Belarus "fell significantly short" of international standards, western observers say.
Belarus has become more isolated under Lukashenko
The referendum approved the lifting of a constitutional ban on a third term for President Alexander Lukashenko.
"Democratic freedoms were largely disregarded by the authorities" said the head of the OSCE observer mission, Tone Tingsgaard.
Opposition demonstrators marched in the capital, Minsk, to condemn the polls.
There were scuffles with police late on Monday, as more than 1,000 people shouted anti-Lukashenko slogans.
The president has run the ex-Soviet republic with an iron grip since 1994.
The Organisation for Security and Co-operation in Europe (OSCE) said the Belarus authorities had "failed to ensure the fundamental conditions necessary for the will of the people to serve as a basis for authority of government".
Both the EU and US earlier questioned whether Sunday's poll was free and fair.
The head of the Belarus electoral commission said preliminary results showed Mr Lukashenko had won 77.3% of the votes and the turnout was nearly 90%.
But an independent survey suggested Mr Lukashenko may not have gathered enough votes to change the constitution.
The poll, commissioned by the Baltic service of Gallup, said that if official turnout figures were taken into consideration less than the required 50% of the electorate could have voted yes.
Belarus has become ever more isolated under Mr Lukashenko, whose rule has been characterised by the closure of opposition media outlets and the prosecution of opponents.
Alongside the referendum, Belarus was also electing a new lower chamber of parliament.
Final results are expected soon.
On Sunday, the BBC was shown what appeared to be evidence of irregularities in the voting.
Some seven million people were eligible to vote
A series of photographs from one polling station showed ballot papers which had already been marked with ticks beside Mr Lukashenko's name before they had been handed out to voters.
In one photo an elderly woman was given a marked ballot on arrival. Another image shows further ballots on a table with boxes already ticked.
The head of the Belarussian electoral commission said the allegations were false.
Other reports said police had detained a number of people conducting an independent exit poll.
The website of the Belarus human rights group Charter 97 said the authorities had "rigged the referendum and the elections".
"Not a single democratic candidate has made it into parliament," it said.
The current constitution limits the president to two terms in office, but Mr Lukashenko has already once extended his rule by means of a referendum. In 1996 he prolonged his first five-year term by two years, to 2001.
He was then re-elected for another five years, in a poll which was criticised as undemocratic by Western observers.
With the next presidential election due to fall in 2006, he claims that a third term is for the good of the nation, but his opponents have accused him of trying to become leader for life.