BURUNDI (Reuters) - Voters in Burundi have overwhelmingly approved a new constitution, the electoral commission said on Monday, ushering in changes that could let the president stay in power to 2034.

An opposition coalition rejected the result of last week’s referendum while the United States said the process had been marred by voter intimidation.

The commission reported that 73 percent had voted in favor of amendments extending the presidential term from five to seven years and allowing President Pierre Nkurunziza to seek two more terms, beginning in 2020.

Commission chairman Pierre Claver Ndayicariye told a news conference that 96 percent of the 4.7 million registered voters cast ballots last Thursday.

Opposition leaders say the changes to the constitution will allow Nkurunziza -- whose party gave him the title of Supreme Eternal Guide this year -- to be above the law.

Evariste Ngayimpenda, a top official of the Amizero y’Abarundi opposition coalition, denounced “a process which was tainted with many incidents including arrests, imprisonments and killings”.

“We reject these results and we will file a complaint because the process was marred by lot of irregularities, even during the counting of the votes,” he said.

Opposition campaign workers had been terrorised and refused access to polling stations, he added.

Rights groups said campaigning and the vote itself took place in a climate of fear and intimidation. The government had denied the vote would be anything but free and fair.

In Washington, the U.S. State Department said the government had allowed vigorous campaigning by the opposition during the designated two-week campaign period. “But numerous cases of harassment and repression of referendum opponents in the months preceding the vote contributed to a climate of fear and intimidation,” spokeswoman Heather Nauert said.

She also condemned Burundi’s suspension of broadcasts by the BBC and Voice of America two weeks before the referendum.

“This decision along with other media restrictions, arbitrary arrests, and harsh sentences for human rights defenders signals continuing limitations on civic and political space in Burundi,” Nauert said.

Burundi ranks 159th out of 180 countries on the World Press Freedom Index compiled by the advocacy group Reporters Without Borders.

Nkurunziza, 54, is a former rebel leader who came to power in 2005 at the end of a long civil war in which 300,000 died. The East African country has broadly the same ethnic make-up as neighboring Rwanda, where 800,000 Tutsis and moderate Hutus were slaughtered by Hutu extremists in a 1994 genocide.

Nkurunziza was due to step down in 2015 but early that year announced he was seeking a third term, triggering deadly clashes with his opponents and a political and security crisis that has gripped the country of 10 million since.