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The former guerrilla turned Africa's longest-ruling leader, now 90, was on his 13th trip to China, seeking more Chinese investment in his nation's stagnant economy.
"The traditional friendship between China and Zimbabwe was forged in the glorious years when we stood shoulder to shoulder against imperialism, colonialism and hegemony," Xi told Mugabe - who is subject to sanctions by the US and European Union - at the Great Hall of the People in Beijing.
"The Chinese people value friendship and we will never forget those good friends and good brothers who have shown mutual understanding and support vis-a-vis China and who have come through thick and thin with us."
The comments by the head of the world's second-largest economy were unusually effusive compared to the usual Chinese diplomatic formality.
He also called the Zimbabwean president a "renowned leader of the African national liberation movement" and "an old friend of the Chinese people whom we respect very much".
Mugabe said he felt "very much at home", thanking Xi for the invitation which reminded him of the past and "brings our past to the present".
Zimbabwe's relations with China and the Chinese Communist Party date back to the liberation struggle of the 1970s, when Beijing provided arms and trained some of the top guerrilla leaders.
Xi and his wife Peng Liyuan greeted Mugabe and his wife Grace - dressed in a vivid yellow and brown African-style outfit - with full military honours, with a band playing the two countries' national anthems as a 21-gun salute was fired and the two presidents inspected a military honour guard.
Zimbabwe's state-owned Sunday Mail newspaper at the weekend quoted Mugabe's spokesman George Charamba as saying the five-day state visit was "largely looking for investment of an infrastructure nature", with the focus on energy and transport.
The two leaders watched as officials signed nine agreements, including some on loans and food donations, but no values were given.
Zhang Ming, vice foreign minister for African affairs, said Xi and Mugabe stressed expanding and improving relations and also discussed broader African and international issues.
"Economically the two sides should continue to seek mutual benefit and common development and be good partners in this regard," Zhang told reporters.
Mugabe has ruled Zimbabwe since 1980, his more than three decades in power starting amid optimism but eventually characterised by corruption and mismanagement leading to hyperinflation and enduring economic crisis, along with brutal crackdowns against political opposition.
In the face of Western opprobrium he adopted a "look east" policy, forging new ties and buttressing existing ones with east Asian countries including China.
There was a partial economic recovery during a power-sharing agreement with the Movement for Democratic Change, which ended last year when Mugabe's Zanu-PF party won elections which the opposition says were rigged.
But independent analysts estimate that unemployment in what was once known as the breadbasket of Africa still stands at 80 percent.
Beijing's diplomatic and economic footprint across Africa has expanded hugely in recent years, as it seeks resources to power its economy. It dismisses concerns over rights abuses, arguing that it does not interfere in other countries' internal affairs.
China invested more in non-financial sectors in Zimbabwe than in any other country on the continent last year, exceeding $602 million, the official Xinhua news agency cited Chinese government figures as saying.
Chinese companies are active in mining, construction, telecommunications and agriculture.
At least two China-linked firms, Anjin Investments and Jinan Mining, have operated concessions at Zimbabwe's hugely valuable Marange diamond fields.
Zimbabwe's military, police and intelligence services are believed to have links to Marange, where overseas campaign groups have raised concerns about rights abuses.
Chinese companies also have interests in platinum and chrome mining.
Mugabe drove Zimbabwe's controversial seizure of white-owned land, and the country has a law requiring foreign firms to hand over 51 percent of their shares to black Zimbabweans. It is not clear how Chinese investors will be affected.
Besides meeting Xi, Mugabe's visit includes talks Tuesday with Premier Li Keqiang. He is also scheduled to visit Xian and Guangzhou.
As well as his wife, Mugabe is accompanied by the ministers for agriculture, tourism, industry and commerce, and finance and economic development, Xinhua reported.
Mugabe is currently chairman of the Southern African Development Community, and is also in line to lead the 54-nation African Union from next year.
But he was the only leader from southern Africa not invited to attend a major US-African summit in Washington earlier this month.