* ECOWAS accuses Guinea of "mass human rights violations"
* Group says violence threat to stability in region
* ECOWAS also threatens full sanctions against Niger
(Adds Nigeria president, paragraph 8)
By Camillus Eboh
ABUJA, Oct 17 (Reuters) - West Africa regional bloc ECOWAS on Saturday imposed an arms embargo against Guinea, accusing the ruling military junta for "mass human rights violations" during anti-government protests last month.
The Economic Community of West African States (ECOWAS) also threatened full sanctions on Niger if President Mamadou Tandja does not take immediate steps to resolve the country's political crisis.
International pressure has increased for Guinea's military leader Captain Moussa Dadis Camara to step down after gunmen used live rounds against protesters in a stadium on Sept. 28. More than 150 people died and thousands more were wounded in the incident, according to a local rights group.
The 15-member regional body said the violence in Guinea posed a "real threat to the peace, security and stability of the region."
"In view of the atrocities that have been committed ... the authority decides to impose an arms embargo on Guinea under the ECOWAS Convention on Small Arms and Light Weapons," said the communique at an ECOWAS heads of state summit in Nigeria's capital Abuja. It was unclear how the group would enforce the embargo.
The United States, France and the European Union have called on Camara to resign and the International Criminal Court said Thursday it was investigating the killings.
Amnesty International issued a statement this week calling on the junta to free people swept up in a wave of arrests after the Sept. 28 violence.
"All steps must be taken immediately to stop the spate of killings of innocent Guineans who are yearning for immediate restoration to constitutional order," said Nigeria President Umaru Yar'Adua, who is also ECOWAS chairman.
The regional body appointed Burkina Faso President Blaise Compaore to lead a mediation team to seek talks between Guinea's political leaders to end the crisis.
West African leaders also raised concerns about the political conflict in Niger, threatening full sanctions if President Mamadou Tandja does not suspend legislative elections next week.
The crisis in Niger pits Tandja, who won an August referendum extending his time in power, against opposition parties and civil society groups.
Some 10,000 people took to the streets of Niamey on Saturday, calling for a boycott of parliamentary polls due to take place on Oct. 20 and warned Tandja of the consequences of not leaving office when his term ends later this year.
The president held a referendum in August to change the constitution and give himself three more years in power.
"Tandja must leave power on Dec. 22 in order to avoid trouble similar to those we saw in Guinea," said Bazoum Mohamed, vice president of the opposition PNDS party, which has been leading the anti-Tandja coalition.
"We demand that the elections are not held to allow a return to constitutional order," he added.
There was a heavy security presence at the demonstration but no reports of any violence.
(Additional reporting by Abdoulaye Massalatchi in Niamey; Writing by Randy Fabi)
(For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ )