* Myanmar president in trade visit
* Cameron urges him to improve record on rights and democracy
President Thein Sein, the first leader of Myanmar to visit Britain in more than 25 years, promised to release all his country's political prisoners by the year's end after his host, Prime Minister David Cameron, pressed him to speed up reforms.
Cameron, who visited the former military dictatorship last year, asked Thein Sein to ensure the constitution was changed to allow opposition leader Aung San Suu Kyi to contest a presidential election in 2015 and urged him to halt inter-ethnic violence against Myanmar's Muslim minority.
"We very much welcome the reform process you are undertaking in your country and look forward to free, fair and open elections in 2015," Cameron told Thein Sein. He was ready to help spur the economic and political transition of the one-time British colony with aid money, his office said in a statement.
Thein Sein, a former military commander, wants the West to help the economy of the former Burma recover from decades of dictatorship, Soviet-style planning and international sanctions, but rights groups say the West should proceed cautiously until he enacts deeper reforms.
Thein Sein said on Sunday that he had disbanded a security force accused of rights violations against Rohingya Muslims in Rakhine State in the west of Myanmar, scene of deadly violence between Muslims and majority Buddhists in the past year.
He has already freed some political prisoners, and in a speech at the Chatham House think tank promised to free all those remaining by the end of this year, saying a special committee was tackling the backlog.
"I guarantee to you that by the end of this year there will be no prisoners of conscience in Myanmar," said Thein Sein.
"Over the last two and a half years, we have embarked upon a transformation which I believe is unprecedented."
He also said he was close to brokering a nationwide ceasefire to end long-running ethnic conflicts.
But rights activists were unimpressed. About 30 members of the campaign group Avaaz protested outside the British parliament with a banner reading: "Cameron - Don't let Burma become the next Rwanda", a reference to the 1994 genocide when hundreds of thousands were killed.
At least 237 people have been killed in Myanmar in religious violence over the past year and about 150,000 have been displaced. Most of the victims were Muslim and the deadliest incidents happened in Rakhine, where about 800,000 Rohingya Muslims live, according to the United Nations.
One activist waiting for Thein Sein outside his central London hotel held a placard that read: "Wanted for War Crimes: President Thein Sein. Do not Reward."
On a two-day visit to talk trade, aid and democracy at a time when mineral-rich Myanmar is opening up its oil, gas and telecoms sectors to foreign investors, Thein Sein was vague about future investment opportunities, mentioning only the tourism and healthcare sectors in broad terms.
Cameron's office said the two men had discussed developing links between their respective armies and "educational partnerships for English language training".
Western leaders have praised Thein Sein for ending the house arrest of opposition leader Aung San Suu Kyi and other reforms, but want him to loosen the military's grip further.
Rushanara Ali, a lawmaker from the opposition Labour party, said Britain's voice could make a difference.
"It is important not to underestimate the soft-power influence that Britain has on the Burmese government. We've got a unique responsibility," she told Reuters.
Thein Sein this year became the first leader of his country since 1966 to visit the White House. After leaving Britain, he is due to travel on to France.