Violence in southern Yemen has marred an election to replace veteran leader Ali Abdullah Saleh.
At least nine people, including a child, were killed in violence which had been widely expected after separatists called for a voter boycott.
Half of Aden's polling stations closed early as a result but voting in the capital, Sanaa, was calm and orderly.
A US spokeswoman said the country was "encouraged" by the "very strong and positive referendum".
"We congratulate the Yemenis for really launching this process, taking ownership of it as a population, and we will stand with them as they take the next steps," state department spokeswoman Victoria Nuland said.
The poll comes a year after violent anti-government protests erupted in the Arabian peninsula's poorest country.
Voters are due to rubber-stamp Vice-President Abdrabbuh Mansour Hadi - the only candidate.
British election observer Baroness Nicholson was in a polling station in Aden, the main city in the south, when an attack happened nearby.
She told the BBC World Service programme Newshour that she and two female Yemeni ministers were monitoring voting when "a great sequence of explosions" took place near to the building.
She said no-one at the polling station was injured and she did not think she had been the target of the attack.
But she said she had been "encouraged" by speaking to people before the vote who told her "we know it's going to be horrible but we're jolly well going to go out and vote".
Elsewhere in Aden two soldiers were killed.
The separatist Southern Movement had announced a day of "civil disobedience" to mark the vote.
A leader of the movement, Abdulhamid Shokri, said four civilians - including a child - had died in the city during clashes between security forces and people opposed to the election.
Officials told BBC Arabic that gunmen killed four soldiers in an attack on a polling station in Hadramawt province.
Two soldiers were also killed in an attack in the al-Hawta district.
Shia rebels in the north also called for a boycott.
The BBC's Rupert Wingfield-Hayes, in the capital Sanaa, says such violence has been seen in previous elections and had been expected.
But Sanaa itself was largely peaceful, says our correspondent, with many people voting amid tight security. In one district officials said 45% of eligible voters had cast their ballot by midday.
"We've seen extensive voting - which we didn't expect - especially from young people," said election official Ammar Al Magbali.
Saleh era ends
The main supporters of the uprising that began in January 2011 are backing Mr Hadi, whose election posters are prominently displayed in Sanaa.
He is from the south himself and has called for dialogue with the separatists.
Yemeni human rights activist Tawakkol Karman, joint winner of the 2011 Nobel Peace Prize, said that the election marked the end of Mr Saleh's 33-year reign.
"We are building the democratic and happy Yemen that all of the youth and women have dreamed about," she said.
But the country still faces many challenges - an ongoing rebellion in large parts of the country, al-Qaeda militants, widespread malnutrition among children and severe drought.
The election follows a deal brokered by Yemen's Gulf Arab neighbours.
Mr Saleh signed the agreement to step down - but only once a new president has been elected.
The new president is due to stay in office for two years, when a further round of presidential and parliamentary elections are scheduled.