WINDHOEK (Reuters) - Namibia's ruling SWAPO party was heading on Tuesday for another big election win, with a two-thirds majority that gives it the power to change the constitution looking likely as more votes came in.

Initial results from just over 10 percent of the 1.18 million registered voters show the South West Africa People's Organisation, a former guerrilla movement that led the arid state to independence in 1990, leading with 70 percent.

The electoral commission said final results from last week's presidential and parliamentary vote would be published on Tuesday but counting has been delayed, and analysts said it may take until late this week to get the final count.

SWAPO's sternest political challenge yet comes from the Rally for Democracy and Progress (RDP), which split from the ruling party in 2007, and holds 10 percent of the votes cast.

"By any standards, it will be a landslide victory ... and although it's too early to say if SWAPO will get the two-thirds, the RDP, by Namibian standards, will be a strong opposition," said Graham Hopwood, director at the Institute for Public Policy Research.

Namibia's politics have been dominated by SWAPO since independence and opposition parties struggled to make an impact.

Altogether, 72 National Assembly seats are up for grabs in Namibia, one of Africa's wealthier states because of diamond and uranium exports. In 2004 elections SWAPO won 55 seats and needs to secure 48 seats to retain the two-thirds majority.

SWAPO has managed to keep most of its traditional support in the central-northern regions, where 60 percent of the population live, but some votes went to RDP.