LISBON (Reuters) - Portuguese riot police detained several ultra-right protesters on Friday after clashes in downtown Lisbon marred celebrations of the 51st anniversary of the Carnation Revolution, which ended a fascist dictatorship.

Extreme-right groups such as Ergue-Te (Rise Up), Habeas Corpus and Grupo 1143 had called a rally, which city authorities banned, to protest ahead of next month's parliamentary election against growing numbers of immigrants.

A few dozen protesters showed up at the Square of Tolerance, which is popular with immigrants from Africa and Asia, just off the Avenida da Liberdade thoroughfare where hundreds of thousands traditionally gather for the Carnation Revolution.

They soon became embroiled in pushing and shoving with anti-fascist activists, leading police to intervene and use batons.

Television footage showed officers in riot gear handcuffing and arresting the leader of Ergue-Te, ex-judge Rui Fonseca e Castro, prominent neo-Nazi activist and Grupo 1143 leader Mario Machado, and another protester who appeared with blood on his forehead.

Machado has previously been jailed for assault, racial discrimination and other crimes, and has another sentence for hate speech and inciting violence to serve.

Around 1.5 million migrants live in Portugal, about triple the number a decade ago.

Portugal's centre-right government has toughened some immigration rules in the past year, reflecting attempts elsewhere in Europe to fend off the rise of the far-right.

Still, the country remains relatively open to migrants, particularly from Portuguese-speaking countries in Africa and from Brazil.

While anti-immigration sentiment is expected to play a role in an early election on May 18, far-right party Chega has been treading water in opinion polls after a surge in the previous election last year when it came third and quadrupled its parliamentary representation.

(Reporting by Pedro Nunes and Andrei Khalip; Editing by Aidan Lewis)