Gnassingbe had hoped to deflect his neighbours' anger over last week's abrupt change of regime by promising to hold presidential elections which would confirm or overturn his rule within 60 days.
But regional leaders were unimpressed and insisted that, by refusing to hand power immediately to the speaker of parliament, the 39-year-old son of the late president Gnassingbe Eyadema was prolonging his unconstitutional "coup d'etat".
" Ecowas chairman President Mamadou Tandja has announced the imposition of sanctions against Togo by the organisation," said a statement released by the 15-nation bloc's Abuja headquarters.
The sanctions include Togo's suspension from the Economic Community of West African States, a regional travel ban on Togolese officials, the recall of west African ambassadors and a complete arms embargo, it said.
US State Department spokesperson Richard Boucher said that Washington supported the Ecowas decision.
"The United States does not accept as legitimate the designation of Gnassingbe as interim president and calls on him to step aside immediately," Boucher said in a statement.
He said the United States had ended all military assistance to Togo and said: "We are reviewing all aspects of our relations with Togo in order to identify further means of supporting the actions of Ecowas."
This slap from the US and from Togo's neighbours and former allies came as 25 000 opposition supporters demonstrated in the Togolose capital Lome against Gnassingbe's rule.
Gnassingbe's father, President Gnassingbe Eyadema, died on February 5 after 38 years of iron-fisted rule over his tiny and poverty-stricken nation.
The military immediately moved to install his son as his successor, brushing aside Togo's 1992 constitution, which was amended in 2002.
On Friday, after a series of meetings with Ecowas officials, Gnassingbe vowed to hold elections.
But he stopped short of handing over power in the run-up to polling day to the speaker of parliament, as demanded by both Ecowas and Togo's constitution.
Nigeria's President Olusegun Obasanjo, the current chair of the African Union, blasted this decision as "unacceptable".
The head of the African Union Commission, Alpha Oumar Konare, also condemned Gnassingbe's decision to cling to power and reiterated calls for a return to "constitutional legality".
Among Africa's main power-brokers, only South African President Thabo Mbeki, who had previously denounced an "unconstitutional charade" in Togo, voiced satisfaction with the election plan.
Opposition supporters said they would organise another march on Wednesday and then continue to do so every Saturday until Gnassingbe steps down.