(CNN) -- Government officials in Alexandria, Egypt, increased security around places of worship following an explosion that killed at least 21 people at a church in the region, the governor told state media Saturday.
"We are stressing now on guarding churches," Adil Labib, the governor of Alexandria, told state-run Nile TV.
Evidence indicates that a suicide bomber caused the blast, the country's Interior Ministry said.
Even though the scene of the explosion was blocked off, protesters carrying crosses gathered near the scene, Nile TV reported.
Authorities believe the bomber was killed in the blast, the Interior Ministry said in a statement. Forensic testing confirmed that the explosive device used was homemade and contained nails and ball-bearings, according to the statement.
Egypt's health ministry said 79 people were wounded in the attack in Alexandria, the country's official Middle East News Agency (MENA) reported. Four of them were police officers posted outside the church to protect the Christians worshipping inside, the interior ministry said.
"We heard a thunderous sound of explosion. I fell on the ground. I saw a car burning. You cannot imagine what we have seen there. ... It was a horrible scene," said Nermin Nabil, who was crossing a street near the church when the blast went off.
Nile TV showed firefighters battling a blaze at the scene.
"This terrorism act has shocked us, hurt hearts of the Egyptians, Muslims and Coptics," said Egyptian President Hosni Mubarak, who addressed the nation hours after the bombing. "I would say with confidence, that we will chase the people who planned and committed this terrorism act, and we will chase the people involved with them."
U.S. President Barack Obama on Saturday condemned the attack.
"The perpetrators of this attack were clearly targeting Christian worshippers, and have no respect for human life and dignity," Obama said in a statement. "They must be brought to justice for this barbaric and heinous act."
"We are continuing to gather information regarding this terrible event," the statement continued, "and are prepared to offer any necessary assistance to the Government of Egypt in responding to it."
A car parked in front of Alexandria's Church of Two Saints exploded shortly after midnight, Egypt's Interior Ministry said. Coptic Christians were attending services there at the time of the blast, Nile TV reported.
The car was filled with explosives, Nile TV reported, citing the interior ministry.
A nearby mosque was damaged, and eight Muslims were among the wounded, the Interior Ministry said.
Mubarak called for a swift investigation of the "criminal act" and urged Egyptians to stand together "in the face of ... terrorism and those who want to disrupt the nation's security, stability and unity of its people," presidential spokesman Ambassador Sulaiman Awad said.
Labib told Nile TV that samples from the scene had been sent to a government lab as part of an investigation.
"The attack targeted all Egyptians and not just our Coptic brethren," Labib said, according to MENA.
Egyptian officials are blaming foreign elements for the attack, MENA reported.
Copts, who are adherents of an Egyptian sect of Christianity, make up about 9% of the nation's population. About 90% of Egyptians are Muslims.
Al-Azhar, one of Egypt's oldest centers of Islamic study and worship, issued a statement condemning the attack.
"This is a criminal act that can never be justified (in) any religion. Islam specifically prohibits any attacks on religious places. As a matter of fact, it tasks Muslims with protecting religious places of worship for Muslims and non-Muslims," Al-Azhar spokesman Mohammed Tahtawi told Nile TV.
In November, a group with ties to al Qaeda in Iraq announced that all Christians in the Middle East would be "legitimate targets."
Shortly after the message was released, Egyptian police sources confirmed that security had been reinforced at churches around the country and additional protection was being provided to the head of the Coptic Church, police said.
Tensions have been running high between Egypt's Muslim majority and minority Christians.
The U.S. Commission on International Religious Freedom said in November that 10 Coptic Christian homes and several businesses were burned and looted in Qena province in southern Egypt following rumors of a romantic relationship between a Christian man and Muslim woman. Security officials imposed a curfew and arrested several Muslims, the commission said.
Journalist Mahmoud Fouly and CNN's Mohammed Tawfeeq, Jomana Karadsheh, Amir Ahmed and Yousuf Basil contributed to this report.