DHAKA : Police fired tear gas and rubber bullets at huge crowds of Bangladesh opposition supporters Saturday to break up a giant protest against the prime minister, with one officer killed and scores of people injured in several hours of violent clashes in central Dhaka.

More than 100,000 supporters of two major Bangladesh opposition parties rallied to demand Prime Minister Sheikh Hasina step down to allow a free and fair vote under a neutral government.

Live footage on the verified Facebook page of the main opposition Bangladesh Nationalist Party (BNP) showed thousands of people running for safety as sound grenades went off one after another and plumes of black smoke rose from the roads.

AFP correspondents said the violence spread in roads and alleys in the centre of the capital as police fired tear gas and rubber shotgun rounds, while the protesters threw stones and bricks.

One officer was killed and more than 100 injured, said Dhaka Metropolitan Police spokesman Faruk Hossain, telling AFP: "The constable was hacked in the head by opposition activists."

The protests by the BNP and the largest Islamist party, Jamaat-e-Islami, were the biggest so far this year, AFP journalists on site said, and marked a new phase in their protests with a general election due within three months.

Hasina -- daughter of the country's founding leader -- has been in power for 15 years and has overseen rapid economic growth with Bangladesh overtaking neighbouring India in GDP per capita, but inflation has risen and her government is accused of corruption and human rights abuses.

At least 20 people were rushed to Dhaka Medical College Hospital, the country's largest, with wounds from rubber bullets, police inspector Bacchu Mia told AFP.

The clashes began in front of the city's largest Catholic church when rowdy opposition supporters fought with sticks and allegedly torched a bus and a police post.

The BNP has called a nationwide strike on Sunday to protest the violence.

"Police and armed ruling party cadres attacked our peaceful rally," party spokesman Zahir Uddin Swapan told AFP.

Months of protests
The resurgent opposition has been mounting protests to press their demands for months, despite the BNP's ailing leader Khaleda Zia, a two-time premier and old foe of Hasina's, being effectively under house arrest after a conviction on corruption charges.

Hundreds of opposition activists were detained in the days running up to the rally, officials confirmed, but her supporters poured into Dhaka on Saturday, crammed into buses despite checkpoints on roads into the capital, and even riding on top of packed trains.

"Vote thief, vote thief, Sheikh Hasina vote thief," chanted the crowd at a demonstration in front of the BNP headquarters.

Student activist Sekandar Badsha, 24, from Chittagong, said: "We demand the immediate resignation of the Hasina government, release of our leader Khaleda Zia and establishing the people's right to vote."

At least 10,000 police had been deployed, officials said. Dhaka Metropolitan Police spokesman Hossain said that at least 100,000 people had joined the BNP rally, while up to 25,000 were at the Jamaat protest near the city's main commercial district -- which had been banned by police.

BNP spokesman Swapan told AFP that there were more than one million people at its rally, which he described as its "final call" for Hasina to resign.

If she does not step down voluntarily -- widely seen as inconceivable -- the party has threatened to call more aggressive protests such as strikes and blockades.

Western governments have expressed concern over the political climate in Bangladesh, where Hasina's ruling Awami League dominates the legislature and runs it virtually as a rubber stamp.

Her security forces are accused of detaining tens of thousands of opposition activists, killing hundreds in extrajudicial encounters and disappearing hundreds of leaders and supporters.