First black man to stand for office in Russia leaves pro-Kremlin party claiming its policies are acting as a brake on democracy

His business as a watermelon farmer and his promise to "toil like a negro" played to the worst stereotypes, but the man nicknamed Russia's Barack Obama seemed to offer a breath of fresh air when he ran for office on a pro-Kremlin ticket two years ago.

Now, Joaquim Crima has dealt a blow to the image of Vladimir Putin's ruling United Russia party by resigning from it with a scathing letter to the Russian prime minister, saying that the party is "acting as a brake on our country's road to democratisation".

Crima, 38, originally from Guinea Bissau, was the first black man in Russia to stand for public office when he put himself forward in district elections in Srednyaya Akhtuba, in the southern Volgograd region.

At the time, he praised Putin as an "excellent person, and a serious figure on the world stage" and said he wanted to follow the premier's example and go into politics with United Russia.

Crima, known locally as Vasily Ivanovich, came a disappointing third in the local poll in 2009, but his progressive electoral promises and his skin colour won him the Obama nickname and extensive coverage in the nationwide press. Putin met him last year and praised him for repairing a village road.

In a letter to the premier published online yesterday, however, the businessman said he was quitting United Russia because it had failed to raise state salaries or cut food prices and was "turning more and more into a party of bureaucrats".

The announcement came at a particularly embarrassing time for Putin, who arrived in Volgograd today to chair a United Russia conference.

Crima told Putin he was particularly disappointed that party bosses had refused last summer to help him send 20 tons of free watermelons to people in villages near Moscow that had been damaged by fire. "In the end they just rotted," he wrote.

To make matters worse, Crima has defected to Fair Russia, a political party set up by the Kremlin that has shown occasional signs of slipping its leash.