BOGOTÁ, Colombia — The crowd gathered here in the capital, wearing white shirts and waving campaign signs, as what had once been Colombia’s largest rebel group opened its campaign for the presidency.

But at a northern port, bombs exploded as a different group continued to attack. Five police officers were killed and more than 40 were wounded in the city of Barranquilla in a bombing at a police station. A faction of the National Liberation Army, the guerrilla organization known as the E.L.N., claimed responsibility.

The contrasting scenes this weekend highlight the challenge Colombia now faces in its road to peace. While the Revolutionary Armed Forces of Colombia, known as the FARC, have signed a peace deal with the government and entered politics, many guerrillas of the E.L.N. seem bent on pressing their long battle against the state.

On Monday, President Juan Manuel Santos of Colombia, whose government had been negotiating with the rebels in Ecuador, said he was suspending the talks.