FACTBOX: What next in Zimbabwe's struggle for power?

(Reuters) - Tensions are rising in Zimbabwe over delayed results of the March 29 presidential election, in which the opposition says it beat President Robert Mugabe.

The ruling ZANU-PF party lost control of parliament for the first time in 28 years in a parallel vote.

An emergency regional summit is to be held in Lusaka on Saturday. Below are answers to some key questions on what could happen next.

WHAT RESULTS ARE KNOWN?

Official results give the opposition Movement for Democratic Change (MDC) 99 seats in parliament, a breakaway opposition faction 10 and ZANU-PF 97. One seat went to an independent.

The MDC said its leader Morgan Tsvangirai won the presidential poll outright. Even though the results remain officially unknown, Justice Minister Patrick Chinamasa said ZANU-PF was preparing for a runoff -- necessary if neither candidate wins more than 50 percent of the first round vote.

Senate results show contested seats split 30-30 between the combined opposition and the ruling party.

Control of the 93-seat Senate will depend on who becomes president, with powers to directly appoint 15 members and strongly influence who gets other positions.

HOW ARE REGIONAL COUNTRIES INVOLVED?

The 14-nation Southern African Development Community (SADC) will hold an emergency summit in the Zambian capital on Saturday. A Zimbabwean ministerial delegation will attend.

Critics say the body is a toothless talking shop, too in awe of liberation hero Mugabe to take firm action. South African President Thabo Mbeki, much criticized at home for not taking a stronger line, led failed SADC mediation last year.

But ruling African National Congress leader Jacob Zuma, who rivals Mbeki as South Africa's most powerful man, has called for the results to be released and has taken a harder line than Mbeki. Tsvangirai has met both of them to discuss Zimbabwe's crisis.

WHY ARE DELAYS SIGNIFICANT?

In past polls, results emerged quickly. This time there is no presidential outcome after almost two weeks. According to electoral rules, a runoff between Tsvangirai and Mugabe should be held within three weeks of the results announcement. The longer the delay, the more time Mugabe has to organize his fightback.

WHAT IS MUGABE'S STRATEGY?

Mugabe had looked badly wounded by the parliamentary defeat and there was speculation he would step down. But strong backing by security chiefs appears to have strengthened the government's resolve to counter-attack.

There are suggestions Mugabe will use presidential powers to extend the interval before a runoff to 90 days.

The MDC has accused Mugabe of deploying pro-government militias including youth brigades and the feared independence war veterans to intimidate MDC supporters. Human rights organizations and the MDC say Mugabe has unleashed a campaign of systematic violence in response to his biggest defeat.

WHAT IS MDC STRATEGY?

The MDC has called a general strike for next Tuesday to demand the release of results.

It has gone to the High Court to try to force the electoral commission to release the results. The court must first rule on whether it has the authority to rule on the MDC application. It was expected to decide on Monday.

The MDC is making frequent statements to keep the Zimbabwe situation high on the world agenda and has appealed for foreign help to end Mugabe's rule.

WHAT WOULD HAPPEN IN A RUNOFF?

The opposition says it would unite behind Tsvangirai, which should, on paper, produce an overwhelming victory based on first round results. But Mugabe's control of state power, security forces and militia could make this much less certain.

WHAT IF MUGABE WINS?

If Mugabe wins a runoff or first round results are revised to give victory to ZANU-PF, this is certain to be rejected by the MDC and some of its supporters could take to the streets. However a Kenyan scenario of prolonged protests and bloodshed seems unlikely because of the power of the security forces.

An outcry in the West and increased sanctions against Mugabe and his entourage would be likely. But such measures have so far had little effect on changing things in Zimbabwe.

WHAT IF TSVANGIRAI WINS?

If Tsvangirai wins and he can resist any violent crackdown by ZANU-PF militants and security forces, foreign powers are expected to flood Zimbabwe with aid to rescue the economy.

Former colonial ruler Britain says it is working with the United States, IMF, World Bank and European Union to prepare a $1 billion recovery plan. One option would be for Tsvangirai to form a national unity government with ZANU-PF moderates.

(Reporting by Barry Moody and Michael Georgy)