The polls - seen as a key step in Pakistan's transition to democracy after years of military rule - had been scheduled for Jan. 8.
Bhutto's Pakistan People's Party believes the government is not sincere in holding fair elections, but the party's central executive committee decided to take part in the polls anyway, committee member Nabeel Gabol told The Associated Press.
The party of former Prime Minister Nawaz Sharif quickly followed suit.
"We will not leave the field open for the king's party under any circumstances," party spokesman Ahsan Iqbal told the AP, referring to the ruling party, which is allied to President Pervez Musharraf.
The opposition alleged authorities were postponing the polls to help the ruling party, amid expectations that Bhutto's group could get a sympathy boost at the polls. The ruling party could also suffer a backlash. Bhutto had accused elements within the group of plotting to kill her, a charge it vehemently denies.
It was not immediately clear if Bhutto's party would pursue threats to take to the streets because of the delayed vote. Earlier, party Sen. Babar Awan warned that the delay may trigger street protests and riots.
The killing of Bhutto, a former prime minister, triggered three days of nationwide unrest that killed 58 people and caused tens of millions of dollars in damage. Bhutto's home province of Sindh was especially hard hit and the army was called on the streets.
Election commission head Qazi Mohammed Farooq said the unrest had made it impossible to hold the polls on time.
"For a few days the election process came to a complete halt," he told reporters. As a result, the poll will be held Feb. 18, he said.
Election officials reported that rioters in Bhutto's home province of Sindh burned down 10 election offices, destroying the voter rolls and ballot boxes inside. The violence also halted the printing and distribution of ballots.
Talat Masood, an independent political analyst, said the delay was "mostly about politics."
"The (election) problems are only confined to a few districts. Musharraf naturally thinks if a hostile parliament comes in he has no future."
The party of Nawaz Sharif, the leader of another opposition party, accused Musharraf of wanting a delay to allow anger over Bhutto's death to evaporate. "Right now they are the target of public hatred" said Ahsan Iqbal, a spokesman for the party.
Sen. Tariq Azim, from the ruling party, said the opposition was "turning a blind eye to realties on the ground" following the assassination, but stressed the ruling party had not asked the election commission for any delay.
Musharraf was due to address the nation later Wednesday.
British Foreign Secretary David Miliband called for the elections to take place as soon as possible.
"If the elections can go ahead in a safe way next week, then obviously they should," Miliband told British Broadcasting Corp. radio. "If they can't, they should only be delayed to another date - we can't have an indefinite postponement."
Since Bhutto's slaying, the government has come under harsh criticism for its security arrangements for her, its claim that an Islamic militant was behind her death and its conclusion that it was the force of the blast and not gunshot wounds that killed her.
The government reiterated Wednesday it was "open" to foreign assistance in investigating the death, but indicated it was unlikely to accept a U.N. committee like the one formed to probe the assassination of former Lebanese Prime Minister Rafik Hariri, as some of Bhutto's supporters are demanding.
Foreign Ministry spokesman Mohammed Sadiq said the two cases were "totally different."
Pakistani troops, meanwhile, killed up to 25 suspected militants in a remote region close to the Afghan border where al-Qaida and Taliban fighters operate. The fighting followed the abduction of four soldiers, said army spokesman Gen. Waheed Arshad and an intelligence official. Security forces sustained no injuries in the clashes, which frequently break out in the area.
The government has blamed South Waziristan-based militant leader Baitullah Mehsud for Bhutto's murder, but he has denied involvement.
Also Wednesday, French Foreign Minister Bernard Kouchner met with Musharraf but canceled a planned trip to Bhutto's hometown to pay condolences, a French Embassy official said on condition of anonymity because he was not authorized to comment. He did not say why.
Nisar Khuro, a senior member of Bhutto's party, alleged the government had stopped Kouchner from traveling there, citing security concerns.
"It seems that Pervez Musharraf is stopping foreign dignitaries from coming for condolences," he said. "This is sad."
Later, Kouchner laid a wreath near the spot where she died.
"I pay respect to the memory of a fighter for democracy and freedom," he told reporters. "Her message and her bravery will never be forgotten."
Bhutto was popular with many Western governments because she was a liberal Muslim prepared to speak out against extremism, but critics in Pakistan take a more balanced view, pointing out her two stints as prime minister in the 1980s and 1990s were marred by allegations of corruption and inefficiency.