A second blast has been reported near a church in northern Egypt, hours after at least 21 people were killed during a Palm Sunday service.
The explosion in front of a church in Alexandria has injured several people, according to state TV.
The earlier blast targeted St George's Coptic church in the city of Tanta, about 130km south-east.
Egypt's Christian minority has often been targeted by Islamist militants in recent years.
The blasts appear to have been timed for maximum impact, as people gathered to mark Palm Sunday.
There was no immediate claim of responsibility and the cause of the blast, just one week before Coptic Easter and the same month as Pope Francis is scheduled to visit Egypt, was not known.
Islamic State's branch in Egypt, which has waged a low-level conflict for years in the Sinai peninsula, has stepped up attacks on Christians in Egypt in recent months.
A suicide bombing at Cairo's largest Coptic cathedral killed at least 25 people and wounded 49 in December, many of them women and children, in one of the deadliest attacks on Egypt's Christian minority in years. Islamic State later claimed responsibility for the attack.
Egyptian security forces had been put on alert in anticipation of attacks.
After the first explosion, provincial governor Ahmad Deif told the state-run Nile News channel: "Either a bomb was planted or someone blew himself up."
General Tarek Atiya, a spokesperson at Egypt's interior minister, told AFP news agency that the blast in Tanta took place near the altar.
A search is underway for any more explosives which could be in the vicinity.
Pope Francis, who is due to visit Egypt later this month, has condemned the attack.
The Pope expressed his "deepest condolences" to all Egyptians and to the head of the Coptic Church, who will host the pontiff on his trip.
A shift in Islamic State's tactics from attacking soldiers and police to targeting Christian civilians has become a potential turning point in a country trying to halt a provincial insurgency from spiralling into wider sectarian bloodshed.
Egypt's Christian community has felt increasingly insecure since Islamic State spread through Iraq and Syria in 2014, ruthlessly targeting religious minorities. In 2015, 21 Egyptian Christians working in Libya were killed by Islamic State.
Copts face regular attacks by Muslim neighbours, who burn their homes and churches in poor rural areas, usually in anger over an inter-faith romance or the construction of church.
Tanta was also the site of another attack earlier this month when a policeman was killed and 15 were injured after a bomb exploded near a police training centre.
- Reuters / BBC