UPDATE 4-Mandela boosts ANC's Zuma in final election rally

* Zuma says ANC will not change constitution

* Mandela, looking frail, attends final rally

* Opposition parties criticise corruption

(Adds COPE comments)

By Tiisetso Motsoeneng

JOHANNESBURG, April 19 (Reuters) - ANC leader Jacob Zuma, boosted by the support of Nelson Mandela at a final election rally, dismissed accusations his ruling party planned to change South Africa's constitution and appealed for national unity.

The ANC is almost certain to win the national and provincial elections on April 22 but faces its biggest challenge since coming to power when apartheid ended in 1994.

Zuma told tens of thousands of supporters who packed two sports stadiums in Johannesburg that the ruling party had used its parliamentary majority responsibly.

"In 15 years that the ANC has been in power, the ANC has never used its electoral mandate to change the constitution and it has no intention of doing so," he said.