The sudden ceremony on Wednesday, which would normally be publicised in advance as a major state occasion, follows a disputed election on August 9 in which Lukashenko claimed a landslide victory.

The opposition accuses him of massive vote-rigging and has staged more than six weeks of mass protests demanding his resignation.

The United States and European Union are drawing up sanctions against officials involved in the election and subsequent crackdown.

The official news agency Belta said Lukashenko placed his right hand on a copy of the constitution and swore the oath of office at a ceremony attended by several hundred people.

Rumours had swept Minsk that the 66-year-old leader, in power since 1994, was preparing for a snap inauguration ceremony when a motorcade swept through the centre of the capital earlier on Wednesday.

An opposition politician, Pavel Latushko, said the inauguration was like a secret "thieves' meeting".

"Where are the jubilant citizens? Where is the diplomatic corps?" he posted on social media. "It is obvious that Alexander Lukashenko is exclusively the president of the OMON (riot police) and a handful of lying officials."

Latushko called for "an indefinite action of civil disobedience".

Lukashenko, taking the oath for a new five-year term, promised to "faithfully serve the people of the Republic of Belarus, respect and protect the rights and freedoms of the person and of the citizen" and defend the constitution.

He has so far withstood the protests with backing from his ally, President Vladimir Putin of Russia.

Despite its population of only 9.5 million, Belarus matters to Russia as a buffer state against NATO and a conduit for Russian exports of oil and gas.

At a summit last week, Putin granted Lukashenko a $US1.5 billion ($A2.1 billion) loan, and the two countries are holding "Slavic Brotherhood" defence exercises in Belarus.

Australian Associated Press