Ugandan police said they believe terrorists are targeting Kampala before national elections this month, the latest in a series of threats since twin bombs killed 79 people in the capital last year.
Police told people living in Kampala to look out for unexpected gifts, flowers, prompting one newspaper to warn readers of a potential plot on Valentine's Day, four days before the February 18 poll.
However, the opposition said it feared the government could use the alert for a clampdown before the presidential and parliamentary elections.
We have received credible information of a plan by terrorists to carry out attacks during the last days of the elections, Uganda's top police officer, Inspector General Kale Kayihura, said in a statement late on Saturday.
Consequently, the police supported by the other sister security agencies are stepping up vigilance to avert these evil plans.
Suicide blasts tore through two Kampala bars on July 11 while people were watching the World Cup soccer final, marking the first attacks on foreign soil by Somalia's al-Qaeda-allied al Shabaab militant group.
Al Shabaab has threatened to carry out more attacks until Uganda and Burundi withdraw their troops from an African Union force protecting a weak United Nations-backed interim government in Somalia.
Analysts widely expect long-serving Ugandan President Yoweri Museveni to win a fourth term despite a strong challenge from former ally Kizza Besigye, who has run twice before.
The opposition, however, have been eating into the president's rural base and say that, if they lose, it will be because of rigging. Besigye has promised to release his own results and warned of Egyptian-style protests if the poll is unfair.
Since the July attacks Uganda has defiantly committed to sending more troops to Somalia. In November Museveni became the first foreign head of state to visit the Horn of Africa country's chaotic capital Mogadishu for almost 20 years.
Opposition officials in Kampala told Reuters they were worried the alert could be an excuse to round them up or ban their rallies during the final days of campaigning.
Police boss Kayihura and Besigye regularly exchange public criticism. Besigye said this week that Kayihura had no experience and called the police a militia.