Sanaa, Yemen (CNN) -- The Yemeni government blamed an affiliate of al Qaeda in the Arabian Peninsula on Tuesday for a massive suicide bombing that killed more than 100 soldiers a day earlier.
Jihadist websites have posted a statement purportedly from the group, Ansaar al-Sharia, claiming responsibility for the attack. But CNN could not independently verify the authenticity of the statement.
The soldiers killed Monday were preparing for Tuesday's National Day of Unification ceremonies in Sanaa when they were attacked. The holiday celebrates the unification of South Yemen and North Yemen on May 22, 1990, to form Yemen.
The parade and festivities continued as planned Tuesday, albeit with a somber message.
"Our hearts bleed ... over the martyrdom of our noble soldiers. The criminal terrorists tried to kill our joy, but we will overcome their barbaric attack, and we will always triumph because we will never allow them to assassinate our dream in building a better nation," military chief of staff Ahmed al-Ashwal told the crowd.
He added, "We assure to you ... that we are at the outskirts of our final battle, God willing, in our fight with what is known as Ansaar al-Sharia so we can restore peace and security to every part of our dear nation."
Monday's explosion appeared to be the deadliest attack ever on troops in Yemen. It left at least 101 dead and more than 220 injured -- some in critical condition, authorities said.
More than an hour after the attack, there were still bodies at the blood-spattered scene. Some ambulances carried six or seven victims each, an official said.
"We heard a massive explosion. Minutes later, there were so many emergency vehicles, it seems as if hundreds were injured," said resident Ali al-Husseini, who was near the attack.
The partially lawless Middle Eastern country has become a central battleground in the fight against al Qaeda. The terror network's leader recently called for an uprising against the new president.
The Yemeni branch of the group calls itself al Qaeda in the Arabian Peninsula; Ansaar al-Sharia is an affiliate of AQAP.
The attack took place about 200 meters (218 yards) from the presidential palace.
President Abdrabuh Mansur Hadi fired top security officials, including a nephew of the former president, after the attack.
Gen. Ammar Saleh was sacked as director of the National Security Bureau, said a Yemeni official who is not authorized to speak to the media. He is being replaced by Maj. Gen. Mohammed Jameh al-Khadar.
Saleh may keep a second and more important post, as first deputy for national security.
The head of central security, Abdul Malik al-Tayyeb, was fired by presidential decree after the attack. He is being replaced by Fadhl al-Qosi.
U.N. Secretary-General Ban Ki-moon condemned the attack and called for the perpetrators to be held accountable.
"The secretary-general calls on all in Yemen to reject the use of violence in all its forms and manifestations, and expects them (to) play a full and constructive role in implementing Yemen's political Transition Agreement," a statement from his office said.
In the United States, chief White House counterterrorism adviser John Brennan called the Yemeni president Monday to convey President Barack Obama's "deep condolences for the cowardly terrorist attack," the White House said in a statement. Brennan also offered U.S. assistance on the investigation.
"Mr. Brennan and President Hadi reaffirmed the unshakable partnership between Yemen and the United States, and President Hadi pledged not to let terrorist acts interfere with Yemen's peaceful political transition," the statement said.
Monday's attack came a day after three American contractors working with Yemen's coast guard were wounded in a shooting in the port city of Hodeida, two local security officials said.
One person has been arrested in the attack, the sources said.
Last week, al Qaeda's leader called for the Yemeni people to rise up against the country's new president, portraying him as the stooge of the unpopular former president, Ali Abdullah Saleh, and the United States.
"So, Ali Abdallah Saleh is gone, and his successor Abed Rabbo Mansour Hadi has taken over," al Qaeda chief commander Ayman al-Zawahiri said in a video posted on jihadist forums.
Saleh, who led Yemen for 30 years, relinquished power last year after an extended popular uprising in a transition agreement that was supported by the United States. Hadi was Saleh's vice president, and al Qaeda has exploited the connection to stir resentment against the new government.
Last year, Ansaar al-Sharia, the al Qaeda offshoot, took over the majority of districts in the southern Abyan province, benefiting from the political turmoil in the country. Numerous military bases were evacuated, making it easier for militant groups to grow in power and territory.
On Sunday, fierce clashes between government troops and al Qaeda fighters left 21 people dead, two local security officials said.
The officials said the violence erupted when hundreds of troops attempted to sweep through areas around the district of Jaar, the main stronghold for al Qaeda in Abyan province.
Al Qaeda fighters fought back, kicking off clashes that continued for nine hours, the officials said. Fourteen militants and seven troops were killed in the fighting, they said.
Government forces have been battling fighters loyal to the local branch of al Qaeda for more than a year but have not made lasting headway.
The United States has carried out dozens of drone strikes against militants in Yemen, including the radical American-born cleric Anwar al-Awlaki, who was killed in September.
U.S. officials said last month they had foiled a plot to bring down a U.S.-bound aircraft with a device that originated in Yemen.
The plot was brought to authorities' attention by a mole who infiltrated al Qaeda, a source in the region told CNN.
The mole works for Saudi intelligence, which has cooperated with the CIA for years, the source said.
Information from the mole also led to a drone strike this month that killed Fahd al Quso, 37, whom the U.S. considered a senior operative of al Qaeda in the Arabian Peninsula.
CNN's Elise Labott contributed to this report.