An explosion at a mosque used by Shia Muslims in the eastern Pakistani city of Lahore has killed at least four people, including a 13-year-old boy.
There have been three such attacks this month alone
A suicide bomber detonated a device as people had gathered for evening prayers at the Husainia Hall mosque in an old part of Lahore, police said.
Two security guards were among those killed - eight people were injured.
A series of attacks blamed on Sunni and Shia militants have claimed more than 70 lives in Pakistan this month alone.
Pakistan has a long history of violence, particularly between majority Sunni and minority Shia Muslims.
Sunday's blast follows a number of deadly attacks on Shias over the past few months.
A man carrying a briefcase tried to force his way into the mosque.
"When security guards stopped him he opened fire and blew himself up, killing two security guards," local police officer Zahir Uddin Babar said.
Local Shia leaders arrived to pacify angry people inside, who for some time, prevented police entering and making investigations.
Lahore, 10 October: 3 killed in mosque attack
Multan, 7 October: 40 killed in car bomb attack on Sunni meeting
Sialkot, 1 October: 30 killed plus suicide bomber at Shia mosque
Karachi, 31 May: Around 20 die in bombing of Shia mosque
Karachi, 30 May: Senior Sunni cleric Mufti Nizamuddin Shamzai shot dead
Lahore, 14 May: Six members of Shia family shot dead
Karachi, 7 May: 15 die in attack on Shia mosque
Police said one person who, according to them, looked suspicious, had been arrested.
But no group has said they carried out the attack - the third this month in Pakistan's Punjab province.
Last Thursday, about 40 people died when a militant Sunni meeting was attacked in the city of Multan. A ban on public meetings was imposed after the deadly car bombing.
At least 30 Shias were killed in a mosque bombing on 1 October in the eastern district of Sialkot.
But until this latest attack, the provincial capital Lahore, and particularly the old walled city where the Husainia Hall mosque is situated, had been relatively peaceful, says the BBC's Charles Haviland in the Pakistani capital, Islamabad.
There appears to be an upsurge in Pakistan's decades-old cycle of violence perpetrated by extremists from these two main branches of Islam, our correspondent says.
Information Minister Sheikh Rashid described the recent spate of attacks as a conspiracy to trigger Sunni-Shiite violence.
"There is no Shia-Sunni rivalry in the country and leaders of both sects know that it is conspiracy by those who want to plunge the country into instability," he said.
Sunni Muslims make up about 80% of Pakistan's 150m people. Most of the rest are Shias.