Alexandria, Egypt (CNN) -- Emotional congregants returned to their church Sunday in Alexandria, Egypt, mourning the loss of fellow worshippers in a bombing a day earlier.

Inside the Church of the Two Saints, grisly reminders of the explosion -- believed to be caused by a suicide bomber -- remained as tearful worshippers lighted candles to honor the dead. Broken glass and debris littered the church's interior and portions of the walls were splattered with blood.

Outside, a heavy police presence guarded the church.

At least 21 people were killed and 97 others injured in Saturday's blast, which occurred shortly after midnight as Coptic Christians were attending services at the church, according to Egyptian government officials. Evidence indicates that a suicide bomber caused the blast, the country's interior ministry said Saturday, though witnesses reported seeing a car parked outside the church explode.

"Why would my son or brother go to celebrate the mass by prayer, not by drinking or doing drugs or anything like that, but by praying in the church, and then this would happen to them at the church gate?" one worshipper said Sunday. "What religion or law, whatever it is, would approve what happened yesterday?"

Another worshipper at the church said she lost three family members in the attack.

"What did we do to them? Nothing! We live together (Copts and Muslims) and nothing happens. How would they do this to us, hurt us and make us orphans?" she said.

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.

A small group of Coptic men held a demonstration against the attack about a block away from the church Sunday.

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.

World leaders, including U.S. President Barack Obama and Pope Benedict XVI, condemned Saturday's bombing.

"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."

But Bishop Makar, the spokesman for the Church of the Two Saints, downplayed sectarian tensions in the region.

"In the beginning, we thought the attack was sectarian, to be honest," he said Sunday. "But now we're quite certain that it has nothing to do with sectarianism especially that we have little sectarianism in Egypt. The vast majority are good and love each other. Here in our area by the way we have no sectarian issues at all."

A nearby mosque was also damaged in the blast and eight Muslims were among the wounded, the interior ministry said.

Egyptian President Hosni 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 Saturday.

Alexandria Gov. Adil Labib told state-run Nile TV that samples from the scene had been sent to a government lab as part of an investigation. Authorities believe the bomber was killed in the blast, Egypt's Interior Ministry said in a statement Saturday. Forensic testing confirmed that the explosive device used was homemade and contained nails and ball-bearings, the statement said.

"The attack targeted all Egyptians and not just our Coptic brethren," Labib said, according to the country's official Middle East News Agency (MENA).

Mubarak has vowed to find the perpetrators of the attack, saying in an address to the nation Saturday that "this terrorism act has shocked us, hurt hearts of the Egyptians, Muslims and Coptics."

CNN's Ben Wedeman contributed to this report.