Protestors chanted "Shame!" and "Fascists!"
Police and protesters clashed briefly and several demonstrators were reportedly detained.
Official figures showed Lukashenka won the support of nearly 80 percent of registered voters for a proposal to lift a constitutional rule limiting him to two terms in power.
In the parliamentary vote, the beleaguered liberal opposition failed to win a single seat in the 110-member lower house.
The head of the monitoring mission from the Organization for Security and Cooperation in Europe (OSCE), Tone Tingsgaard, said the "vote fell significantly short of [international] standards."
The U.S. State Department spokesman Richard Boucher also echoed doubts expressed by the OSCE.
Russia was more supportive of the vote, saying monitors from Commonwealth of Independent States (CIS) had found no serious law violations.
Russia's Foreign Ministry said today that the referendum was fair and reflected the will of the people, and should therefore be respected.
The ministry said in a statement the referendum and simultaneous legislative elections were held "in a calm and transparent way" with no serious violations.
In Prague, former Czech President Vaclav Havel opened an international conference by calling the government in Minsk "the last European dictatorship regime."
"We saw yesterday a clear illustration of how totalitarian or authoritarian regimes loathe civic society," Havel said. "I am talking about Belarus -- the election and the referendum in Belarus and the situation [in that country] in general."
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