DHAKA (Reuters) - At least 34 people have died in Bangladesh and scores have been injured, most of them in firebomb attacks, amid rising political unrest fueled by a stand-off between Prime Minister Sheikh Hasina and the main opposition leader.

The renewed political turmoil could cause a delay in shipments by the country's $24-billion garment industry, already under pressure after a string of fatal accidents.

Begum Khaleda Zia, whose opposition Bangladesh Nationalist Party (BNP) boycotted the election on Jan. 5 last year, has demanded that Hasina and her government step down for a new vote under a caretaker administration.

Hasina has refused, instead tightening her grip by arresting key opposition leaders and clamping down on critical media as anti-government protests spread. The violence has worsened sharply since Jan. 5, the first anniversary of the vote.

Police said at least 25 people have died in arson attacks, including two on Friday. Eight more were killed in clashes with police, and one died following injuries from a crude bomb blast, they added.

At least 50 people were injured, some critically, after opposition activists firebombed several vehicles in the capital, Dhaka, and surrounding districts, police and witnesses said.

In Dhaka, at least 29 people suffered burns after attackers hurled petrol bombs at a bus, police said.

"Nine are in critical condition," said Mohammad Sajjat Khandakar, a doctor at Dhaka Medical College Hospital, which has been struggling to deal with growing numbers of the injured.

More than 7,000 opposition activists have been detained since the anniversary, Industry Minister Amir Hossain Amu, the head of a government law and order panel, has said.

The opposition called for another 36-hour countrywide strike from Sunday morning to protest against the arrests and "oppression" of its leaders during an indefinite transport blockade it launched.

Khaleda called the blockade after she was prevented from holding a mass rally in Dhaka on the Jan. 5 anniversary.

Legal action could be considered against Khaleda for ordering the killing of innocent people, said Health Minister Mohammed Nasim.

"They should immediately stop the killing of innocent people, children, woman, laborers," he told reporters on Saturday after visiting the burns victims.

BNP leaders were not immediately available to comment on the threat of legal action.

Hasina and Khaleda have alternated as prime minister for most of the past two decades in a fierce rivalry marked by periods of widespread political violence.

The United States, the European Union and Britain have voiced concern and urged all Bangladeshi parties to engage in dialogue.

(Reporting by Ruma Paul; Editing by Clarence Fernandez)