(Updates with background, adds byline, edits)

By Aung Hla Tun

YANGON, Sept 25 (Reuters) - A suspected bomb exploded outside City Hall in Myanmar's main city on Thursday, wounding four people the day before the anniversary of a bloody military crackdown on major anti-government protests.

"It seems to have been a small bomb, but we are still carrying out investigations," a policeman, who did not want to be identified, told Reuters at the scene of the central Yangon blast.

Those wounded were a man and three women. The officer said they were not thought to be seriously hurt.

Armed police and soldiers immediately sealed off the area, a focal point of the massive protests a year ago led by Buddhist monks against decades of army rule in the former Burma.

At least 31 people were killed in the crackdown, which drew worldwide condemnation when it was launched on Sept. 26, 2007.

With the anniversary looming, security in the former capital has been unusually tight, with armed police and troops patrolling the streets and setting up vehicle checkpoints.

The area around City Hall and the Sule Pagoda, where the daily protest marches ended, has been under particularly tight surveillance by dozens of police and plain-clothes security agents.

The junta, the latest face of 46 years of unbroken military rule in the southeast Asian nation, arrested at least 3,000 people in the crackdown and its aftermath.

Human rights groups say as many as 700 people remain behind bars, although the military authorities say all but a few dozen have been released.

Earlier this month, female activist Nilar Thein, a student leader in a brutally crushed democracy uprising in 1988 and an organiser of the 2007 protests, was detained after a year on the run.

She went into hiding, abandoning her four-month-old daughter, when her husband was arrested in August last year for helping stage the small fuel and food price protests that snowballed into the monk-led demonstrations a month later.

The detention of the still-influential 1988 uprising leaders -- the so-called "88 Generation Students" -- makes any demonstration to mark the anniversary inside Myanmar extremely unlikely. Events are planned for outside the country.

News websites run by exiled dissidents, most of them in neighbouring Thailand and India, have come under cyber-attack in the past week in what they say is an attempt by the generals to prevent coverage of the demonstrations.

Small bomb blasts are common in Myanmar. The military government routinely blames them on underground dissidents or the ethnic guerrilla groups that have been fighting in some cases almost since independence from Britain in 1948. (Writing by Ed Cropley; Editing by David Fogarty)

AlertNet news is provided by