Zimbabwean opposition leader and presidential candidate Morgan Tsvangirai was on the presidential campaign trail again Thursday after his release from police detention for about eight hours on Wednesday in Lupane, Matabeleland North.

Campaigning in rural communities near Plumtree, Matebeleland South, Tsvangirai told VOA he he would not let the incident derail his campaign for president. Tsvangirai will go head to head with President Robert Mugabe in a run-off ballot on June 27.

But human rights groups voiced concern that Harare is tightening its grip on the nation with about three weeks to go to the election, already seriously tarnished by a wave of pervasive and increasingly deadly political violence targeting the opposition.

Amnesty International issued a statement saying Tsvangirai?s detention was part of a ?sudden?and dangerous crackdown on political opposition" in the election run-up.

At the U.S. State Department, spokesman Sean McCormack called on Zimbabwe's regional neighbors including South Africa, to exercise "the maximum amount of leverage" on the Harare government given the rising tide of violence.

Kenyan Prime Minister Raila Odinga called Mugabe a "dictator" and criticized African leaders who have been reluctant to tackle him on human rights.

Nairobi-based political analyst Brian Kagoro told reporter Blessing Zulu of VOA's Studio 7 for Zimbabwe that Mugabe is using terror as a campaign tool.

More reports from VOA's Studio 7 for Zimbabwe...