KAMPALA, Uganda — Ugandan police announced Tuesday that they had found an explosives-laden vest at a popular nightclub here in the capital, thwarting another potential bomb attack just days after three deadly explosions ripped through crowds watching the World Cup championship game.

The authorities also said they had arrested four foreign nationals in connection with Sunday’s attacks, which left at least 76 people dead and raised concerns about the growing reach of the Somali militant group, the Shabab, which took credit for the bombings.

The police said the vest was rigged to a detonator that appeared to be a cellphone, and had been left in a bag in the middle of a nightclub in Kampala.

“One would think that someone had just left their laptop or something,” said Judith Nabakooba, a spokeswoman for Uganda’s police.

Eyewitnesses said they noticed the bag when they had heard a cellphone ringing from inside it, then called the police when they saw what was inside. Authorities said bomb experts had found the vest to be “active,” before neutralizing it.

The authorities said a fourth attack had been narrowly avoided.

Streets in Kampala were cordoned off Monday night and into Tuesday morning, as rumors of another potential attack swirled through the jittery city.

“We are mobilizing members of the public and putting them on high alert,” Ms. Nabakooba said. The Shabab, which is fighting a fierce insurgency against Somalia’s government, has threatened to continue carrying out attacks in Kampala, as well as in the Burundian capital of Bujumbura.

On Tuesday, an official from the group who identified himself as Yonis, an assistant to the Shabab spokesman, told Reuters that Sunday’s attacks had not been carried out by suicide bombers, but by planted explosives.

Ugandan police on Tuesday refused to give any details about the four suspects arrested Monday night, or say how they were connected to the attacks .

The arrests and discovery of the explosive-laden vest have raised tension and security in this East African capital to new highs, and could have larger ripple effects on security networks throughout East Africa.

Uganda is both geographically and religiously on the frontier of the Islamist movement in the Horn of Africa. While Uganda is a heavily Christian country, Kampala has a large and significant Muslim population, including Somalis, Ethiopians and Eritreans.

Securing the country’s borders could be made more difficult by the recently-inaugurated East African Common Market, which has liberalized the movement of goods and people throughout five East African countries, including Uganda. Police officials said that they would be on the lookout for “suspicious” people in Kampala, and said that everyone should carry identity cards wherever they go.

Although the police said they do not want to cast blame on Uganda’s Somali community, some Somalis in Uganda said a backlash against them had already begun, with the police questioning many Somalis and beginning to turn away Somali asylum seekers.

Ugandan police officials confirmed the freeze on refugees, and said it could go on indefinitely.

“Now we are on full alert,” Ms. Nabakooba said. “It could stay for a long time.”

Compared with neighboring Kenya, which shares a land border with Somalia, Uganda has relatively lax restrictions for Somali refugees and admits about 15 each week, said Andi Ali Jama, a leader of the Somali community in Uganda, which denounced the attacks.

“We don’t know what will happen next,” Mr. Jama said. “There is all sorts of sadness.”