text: The call comes after at least four people were killed in clashes between police and protesters.
  Pascal Tsaty Mabiala of the PanAfrican Union for Social Democracy wants to stop Sunday's vote going ahead.
  President Denis Sassou Nguesso has been in power since 1997.
  The vote is aimed at pushing through changes to the constitution such as scrapping age and two-term limits.
  Police fired shots and tear gas in the capital, Brazzaville, on Tuesday in a clamp down on protests against the president's bid for a third term.
  Texting and internet services were cut and public meetings banned ahead of the referendum, residents said.
  Other journalists told the BBC that most shops in Brazzaville were shut, and people were staying at home amid fears of violence.
  "People are demonstrating across the city. The police are firing tear gas bombs," Tresor Nzila, head of the Congolese Observatory of Human Rights, told the Reuters news agency.
  "In certain places, the police have fired warning shots with live fire."
  The opposition have been campaigning under the slogan "Sassoufit", a pun on the French expression for "that's enough".
  Africa's longest-serving leaders:
  The arrogance of power
  The Mobutu and Gaddafi effect
target: An opposition leader in Congo-Brazzaville has called for a "peaceful uprising" ahead of Sunday's referendum on whether the president can run for office again, AFP news agency reports.