Bangladesh police on Saturday arrested seven opposition party members blamed for an alleged pre-election arson attack on a packed commuter train that killed four people and injured another eight.

Friday night's blaze engulfed the intercity Benapole Express in central Dhaka, with hundreds scrambling to pull passengers from burning carriages.

It was the latest in a series of fires to hit railway services since late last year, blamed by police on "deadly acts of sabotage" by the opposition Bangladesh Nationalist Party (BNP) ahead of Sunday's national election.

Police said Nabiullah Nabi, a senior BNP official in Dhaka, and six other party activists were arrested in the capital early Saturday.

"Nabi funded and masterminded the attack," Dhaka Metropolitan Police spokesman Faruk Hossain told AFP by phone.

The BNP and dozens of other opposition parties are boycotting Sunday's vote, which they say is a "sham" designed to entrench the rule of longtime Prime Minister Sheikh Hasina.

The party has vehemently rejected allegations of responsibility for the fires and demanded an international probe into the incidents.

"These are pre-planned acts of sabotage by government functionaries aimed at discrediting the non-violent movement of the BNP," party spokesman A.K.M Wahiduzzaman told AFP.

"Tarique Rahman, our party chief, had expressed his fear that government was hatching these conspiracies to divert people's attention away from the sham election."

Police had earlier lowered the fire's death toll from five to four.

Samanta Lal Sen, a senior official of the Dhaka hospital treating victims of the blaze, said eight people had been critically injured.

Bangladesh foreign minister A.K. Abdul Momen described the latest train fire as "an unforgivable crime against humanity".

"The timing of this tragedy... shows an absolute intention to hinder the festivity, safety and security of the democratic processes," he said.

'Unprecedented'

Quamrul Ahsan, chief of the country's state-run railway authority, told AFP that 32 passenger trains had been suspended over the weekend to "provide more safety to other passengers".

Ahsan said there had been at least four arson attacks targeting the trains ahead of the election.

"These incidents are unprecedented," he said, adding that authorities had boosted security in some inter-city trains.

Hasina, 76, is assured of a fifth consecutive term in Sunday's vote, which observers have criticised as one-sided.

Opposition parties held a series of protests last year demanding Hasina's resignation in favour of a neutral caretaker government to oversee the election and ensure its integrity.

Violence in the run-up to Sunday's vote has killed at least 15 people, and the BNP has called a general strike over the weekend to protest the poll.