A spate of political killings during the current presidential campaign drove hundreds of thousands of Colombians to take part in nationwide demonstrations Tuesday to demand an end to the violence.

Industry halted for half an hour before noon while Colombians stood in streets and squares waving white handkerchiefs, napkins, even sheets of paper. Others honked their car horns to show their frustration with the violence.The demonstration was originally planned to observe the one-year anniversary of the killing of two human rights workers. But it coincided with a dizzying increase in political killings of civilians, human rights workers and political figures around the country.

No arrests have been made in the attacks, which have left more than 50 people dead and dozens missing in the past month.

Some here consider the killings an inevitable companion to elections in Colombia, a tactic used to influence voters. So far, the killings have not increased support for the candidate most associated with a hard-line answer to the violence, a retired former chief of the armed forces, Harold Bedoya, whose backing has dropped from 20 percent to 7 percent in polls.

Others assert that the killings are more likely meant to weaken leftist insurgents before expected peace talks. Such talks have become the top issue as the first round of the presidential elections on May 31 approach.

Luis Eduardo Rodriguez, 56, a construction inspector, carried a white balloon down a street in northern Bogota at noon. With a 20-year-old son in the army, he said he worried daily about the violence that has ground on for more than 30 years.