Nazarbayev has ruled Kazakhstan unchallenged since the fall of the Soviet union [Reuters]

Kazakhstan's President Nursultan Nazarbayev has been sworn in for a new five-year term after securing a landslide victory in an election boycotted by the opposition and criticised by international observers.

In the inauguration ceremony on Friday, Nazarbayev vowed to implement reform and said the election was a vote for stability and prosperity in the Central Asian nation.

"Today is a special day for me. The Kazakhs have made their historic choice and voted for me and I express my thanks," Nazarbayev said at the ceremony in the new capital Astana, whose building he personally masterminded.

In a nod towards criticism that his two-decades rule has provided stability but little semblance of democracy, he promised to increase the powers of parliament and government.

"We will continue our work for the further democratisation of society," he said.

"We must find an optimal solution to widen the powers of the parliament, the responsibilities of the government and to perfect the electoral system."

He said that the world "does not stand in one place" and Kazakhstan needed to modernise and pursue a policy of innovation.

The Organization for Security and Co-operation in Europe said Sunday's election was marred by numerous cases of ballot box-stuffing, voter intimidation and a lack of transparency.

The three rival candidates all openly expressed their support for Nazarbayev and the opposition argued they had been placed in the field by the government to make the vote look legitimate.

Nazarbayev has led the oil-rich ex-Soviet republic unchallenged since independence in 1991.

Kazakhstan's cabinet offered its resignation on Friday, in line with the constitution. Nazarbayev is expected to re-appoint or replace government ministers over the next few days.