AFP - Hundreds of thousands of hardline Islamists demanding a new blasphemy law on Sunday blocked highways and fought running battles with police, leaving four people dead and hundreds injured in the Bangladeshi capital.

Chanting "Allahu Akbar!" ("God is greatest!") and "One point, One demand: Atheists must be hanged", activists from Hefajat-e-Islam marched along at least six highways, blocking transport between Dhaka and other cities and towns.

Police officials told AFP that about 200,000 people had marched to central Dhaka, where fierce clashes erupted between thousands of rock-throwing protesters and security officials, with police beating back demonstrators with batons.

"At least 100,000 protesters" blocked the road at Tongi town, which connects Dhaka with the northern region, local police chief Ismail Hossain told AFP.

Witnesses said rioting broke out after police tried to intercept stick-wielding protesters, most travelling from remote villages, in front of the country's largest mosque. Trouble then spread to central districts of Dhaka.

"This government does not have faith in Allah. This is an atheist government, we will not allow them to live in Bangladesh. Muslims are brothers, we must protect Islam," one protester, filmed by AFP, was seen chanting.

Police fired rubber bullets from armoured vehicles at protesters, who went on the rampage, torching a police office, scores of vehicles and shops, attacking government offices and beating policemen.

Dozens of small bombs exploded, leaving smoke hanging in the air around the mosque.

One policeman suffered serious head injuries after he was beaten by protesters, according to an AFP correspondent at the scene.

Of the dead at least three were taken to Dhaka Medical College Hospital, police inspector Mozammel Haq told AFP, adding almost 100 more had been injured.

M. Adnan, an emergency doctor at the private Islami Bank Hospital, told AFP one protester died and nearly 300 protesters were treated in its two branches.

A senior police officer who declined to be named told AFP between "150,000 and 200,000 demonstrators" marched to Motijheel, Dhaka's main commercial district, where they continued to rally until 10:00 pm (1600GMT).

Deputy Commissioner of Dhaka police, Sheikh Nazmul Alam, said police fired rubber bullets to disperse unruly demonstrators.

The protest was staged as the country was recovering from its worst industrial disaster, which saw at least 620 people killed when a factory building collapsed just outside the capital on April 24.

Hefajat, a newly created radical Islamist group, is demanding the death penalty for all those who defame Islam.

It said it staged the mass protest to push a 13-point list of demands, which also include a ban on men and women mixing freely together and the restoration of pledges to Allah in the constitution.

Hefajat leaders have threatened to launch a campaign to oust the government unless their demands are met.

Marchers blocked highways at Jatrabari and Demra, cutting the city off from the northeast and southeast, including from the main port of Chittagong.

They also blocked roads and bridges in Kadamtali and Hasnabad, severing Dhaka's road links with the south.

The rally was the latest in a series of mass actions by Hefajat, unusual in Bangladesh because of the large numbers of people taking part.

Last month activists organised a general strike as well as a gathering of hundreds of thousands of activists demanding a blasphemy law, in what experts said was the country's largest political rally in decades.

Critics have branded Hefajat's demands a charter for turning Bangladesh into a country like Taliban-ruled Afghanistan.

Women workers including female garment labourers have vented their anger at the group's call to segregate the sexes.

Prime Minister Sheikh Hasina, who has been leading a secular government in the Muslim-majority country since 2009, has rejected the demand for a blasphemy law.

Hasina's ruling Awami League party has accused Hefajat, which draws support from the country's tens of thousands of Islamic seminaries, of being a pawn of the opposition, which lent moral support to Sunday's blockade.

Hardline Islamist groups accuse Hasina's government of trying to intimidate the opposition through a series of trials for war crimes allegedly committed during the 1971 war of independence.

Three Islamists have so far been convicted and two of them were sentenced to death. More than 100 people have been killed during protests over the trials since January this year.