A bomb exploded early Wednesday outside an office building housing a Citibank (search) branch, blowing out windows and causing some damage but no injuries, police said.

The so-called Group of People's Combatants (search), or GCP by its initials in Spanish, claimed responsibility for the attack. An e-mail sent to The Associated Press, claiming to be from the group, said the blast was in opposition to free trade talks between Ecuador, Peru and Colombia with the United States aimed at opening tariff-free access to U.S. markets.

The 10th round of negotiations are now taking place in this Pacific coast city, the country's banking and financial hub.

In recent years, the GCP, described by authorities as a band of leftist extremists, have exploded several small bombs in Ecuador's main cities.