* police say small march demanding poll boycott is illegal * Bouteflika seeks fourth term in April 17 election * opposition says veteran leader too ill to run again By Lamine Chikhi ALGIERS, march 12 (Reuters) - Algerian police prevented opposition leaders from marching on Wednesday to demand a boycott of next month's election, in which President Abdelaziz Bouteflika is seeking a fourth term in power. Bouteflika, 77, registered his candidacy for the April 17 vote last week, one of the few times he has spoken in public since suffering a stroke last year that has raised opposition questions about his ability to govern. opposition leaders, including from the secularist party Rally for Culture and Democracy (RCD) and the Islamist Movement for Peace and Society (MSP), believe Bouteflika's decision ends fair competition in the election. It is a peaceful march, all we want is to convey a message that Bouteflika is too old, too ill to rule Algeria," said Abdelkader Ait Ali, one of those who tried to take part.