Pakistani police said on Wednesday that they had arrested three men suspected of planning a suicide attack near Islamabad, while a suspect from an Al-Qaeda-linked group was held in Karachi.
Pakistan has been hit by a wave of deadly bomb attacks in recent weeks, and security is at unprecedented levels amid fears of more violence from Taliban insurgents avenging a military campaign against them.
"Police have arrested three suspected terrorists with explosives and suicide jackets in Rawalpindi," said a senior police official in the garrison city neighbouring the capital Islamabad.
Another police official in Rawalpindi confirmed the arrests and said the three men were apparently planning to carry out attacks in the city.
Neither official released further details or the identities of the men, saying it would jeopardise the investigation.
In the southern economic of hub of Karachi, police raided a house and arrested a man suspected of planning an attack in the port city.
"We arrested one man on Wednesday morning and recovered two kilograms (four pounds) of explosives and a gun from him," police official Raja Umer Khitab told AFP.
"He belongs to banned sectarian group Lashkar-e-Jhangvi," he said, adding that another suspect had escaped.
Lashkar-e-Jhangvi is regarded as the fiercest of Pakistan's Sunni extremist outfits with links to Al-Qaeda, and is known to have mounted numerous attacks on the minority Shiite Muslim community since it was formed in 1996.
The detainee, identified as Irfan Islam, was involved in "several sectarian killings" and had contacts with militants in the lawless tribal region of North Waziristan on the Afghan border, Khitab said.
Police have stepped up surveillance across the country after the Tehreek-e-Taliban Pakistan (TTP), led by warlord Baitullah Mehsud, threatened to attack major cities in retaliation for the military offensive against militants in the northwest.
Pakistan launched its northwest push after the Taliban advanced to within 100 kilometres (60 miles) of Islamabad in early April, violating a deal to put three million people under sharia law in exchange for peace.
The army on Tuesday announced that it would expand the Swat valley campaign and launch a second front against Mehsud and his network in the rugged, semi-autonomous tribal belt bordering Afghanistan.
More than 1,995 people have died in Taliban-linked attacks here since July 2007, with nearly 170 killed since the military launched the offensive in strikes targeting key cities including Islamabad, Peshawar and Lahore.