European election observers have said Kazakhstan's parliamentary election fell short of international standards.
President Nazarbayev cast his vote in the capital, Astana
The Organization for Security and
Cooperation in Europe (OSCE) said the nation's media had been biased, and the election had not been "transparent".
The only opposition politician in government, Information Minister Altynbek Sarsenbayev, also said the vote had been falsified, and resigned.
Early results put President Nursultan Nazarbayev's party far ahead of others.
The OSCE said it was concerned about a ruling that prevented two opposition leaders from running in the election, and a last-minute decision to use both electronic and paper voting, which it said created confusion in polling stations.
Robert Barry, head of the long-term observation mission, said the "central election commission has functioned in an arbitrary, selective and non-transparent manner".
A written statement from the monitors also said: "Considerable pressure was placed on voters, especially by local government officials and by supervisors in the workplace."
However, the observers also said they had noted "some improvements" from previous elections, notably with regard to media and voter registration.
EARLY RESULTS (18% OF VOTES COUNTED)
Ak Zhol: 16.1%
Democratic Choice / Communists: 6.2%
"Domestic observers were given more legal rights, which provided
them with greater access to the process," the statement said.
On his decision to resign, Mr Sarsenbayev said: "I find it impossible to be a member of the cabinet and
part of the executive branch that has actively interfered
in the election process, and participated in falsification
of results of the people's votes.
Mr Sarsenbayev, co-chairman of the opposition Ak Zhol party, was appointed in July by President Nursultan Nazarbayev in an apparent attempt to appease the opposition.
Another Ak Zhol co-chairman, Oraz Zhandosov, claimed his party had won, and that the authorities were stealing their votes.
Nazarbayev family success
By Monday afternoon, only electronic ballots had been
tabulated - 18% of all votes cast.
Mr Nazarbayev's Otan Party was leading with 42.7% of the vote, and the
Asar party, run by his daughter Dariga Nazarbayeva, was
next with 19.5%.
Ak Zhol had 16.1% of the vote, and the pro-government AIST bloc of the Agrarian and Civic parties had 8.4%.
The radical opposition bloc of the Democratic Choice of Kazakhstan (DCK) and the Communists had about 6.2%.
Many observers have questioned whether the Asar party is genuinely independent of the president's camp.
Some have suspected Ms Nazarbayeva's entry into politics earlier this year was designed to pave the way for a dynastic succession.
But Ms Nazarbayeva says that while she fully supports the president, she set up her own party to break the hold of the power elite which surrounds her father.
Our correspondent in Astana, Ian MacWilliam, says younger people who might normally have voted for Otan opted for the smaller Asar party.