Thousands of police and troops guarded key Western installations and blocked the roads to the US consulate in Karachi on Sunday after a blast killed one person and wounded nine.
Security forces were also checking vehicles for explosives after suspected extremists blew up a stolen car packed with explosives in a district of popular clubs late on Saturday.
The blast occurred in a parking area not far from a club where hundreds of people were gathered for a concert by Indian singer Sonu Nigam.
Syed Kamal Shah, police chief of Sindh, said Harkat-ul Mujahideen al Alami was one of the suspects.
Al Alami is a splinter faction of Harkat-ul Mujahideen, which is fighting Indian rule in the occupied Kashmir region. A senior police official, who spoke on the condition of anonymity, said the pattern of the bombing showed that al Alami was behind the attack.
"The group is infuriated because several of its key members have been arrested. The group is going for the soft targets now," the official added.
Police have blamed al Alami for most of the high-profile terror attacks in Karachi during the last two years, including June 14, 2002 bombing outside the US Consulate that killed 12 Pakistanis.