At least three people have been injured in an explosion in central Yangon, police and witnesses in the former Myanmar capital have said.

The suspected bomb blast occurred on Thursday morning outside Yangon's City Hall and comes on the eve of the first anniversary of last year's bloody crackdown on anti-government protests.

"It seems to have been a small bomb, but we are still carrying out investigations," a police official told Reuters.

None of the injured is thought to have been seriously hurt.

The scene of the blast was quickly sealed off by armed police and soldiers.

The area became a focal point of last year's protests that saw tens of thousands of Buddhist monks and their supporters take to the streets against decades of military rules.

After days of escalating demonstrations the government launched a crackdown on September 26 in which at least 31 people died.

Hundreds more were arrested or remain unaccounted for.