Police and witnesses said opposition activists set ablaze an office of the ruling Bangladesh Nationalist Party (BNP) and stormed another in the old Dhaka area.
BNP activists responded by taking on the attackers. Some 25 vehicles including buses were damaged on the streets as the fighting spread, witnesses said.
One police officer was injured, they added.
Prime Minister Begum Khaleda Zia's hopes for re-election have been hit by the formation of a new movement by disgruntled members of her ruling BNP.
Leaders of the newly formed Liberal Democratic Party (LDP) said they might work with the mainstream opposition alliance trying to defeat the BNP in the coming polls.
The LDP is led jointly by retired army colonel Oli Ahmed, who formally broke from the BNP on Thursday, and former president A.Q.M. Badruddoza Chowdhury, who left years ago over differences with Khaleda and launched his own party, Bikolpa Dhara, which has now merged with the LDP.
Other defectors include former speaker of parliament Sheikh Razzak Ali and a host of current and former ministers and members of parliament.
The defections sparked violent protests by the BNP across the country.
Activists torched the home of Sheikh Razzak Ali in Khulna, 350 km (220 miles) from Dhaka, after he switched sides, and a house of former BNP minister Redwan Ahmed, was set ablaze in Dhaka, police and witnesses said.
BATTLE LINE DRAWN
The main opposition Awami League said it planned to paralyse ports and block highways from Saturday in protest against the government's refusal to reconsider its choice of a caretaker administration to supervise the election.
Khaleda is due to hand over power to the caretaker authority on Saturday at the end of her five-year rule.
"We will cripple the administration," Awami general secretary Abdul Jalil told reporters a day after the ruling BNP said it would stick to its choice of a former chief justice to head the interim administration, despite opposition charges that he had past links with the ruling party.
Police have ordered an indefinite ban on the assembly of people carrying sticks or lethal weapons into Dhaka. The opposition said it would defy the ban.
"We are bracing for a serious law and order challenge as both the ruling and opposition alliances will try to occupy the streets of Dhaka from Saturday," said a senior police officer.
"We are massing extra force as the violence may intensify during the transfer of power to the interim authority," said the officer who asked not to be named.
Police said both BNP and Awami League had called for rallies in the capital on Saturday "at the same time, at the same venue" -- raising fears of fatal clashes. "I am afraid the battle line has been drawn," said a Dhaka city corporation official.
FAMILY DYNASTY
Political analysts and opposition politicians said the emergence of a breakaway group was a blow to Khaleda and would have a strong impact in her fight against former prime minister Sheikh Hasina in the election.
"The LDP has been created to fill up a political vacuum created due to failures of the mainstream parties," analyst Debapriyo Bhattacharya said.
Members of the new group said they had left the BNP because it had become "a family dynasty" that promoted corruption.
Khaleda's eldest son, Tareque Rahman, is the BNP's first joint secretary-general, the number three position in the party, and is being tipped as Khaleda's successor.
Critics have also accused the ruling party of not having done enough to contain the rise of Islamist militant groups in the country, blamed for a wave of bomb attacks last year.

Copyright Reuters, 2006