UK Foreign Secretary William Hague has visited Tunisia as part of a tour of North African and Middle Eastern states affected by calls for political change.
After meeting interim Prime Minister Mohammed Ghannouchi, he said there was an opportunity for political openness and economic development in the region.
He also offered Tunisia assistance to help build democratic institutions.
Later he arrived in Jordan where he will meet King Abdullah II and discuss the recent events in Egypt and Tunisia.
The Foreign Office said Mr Hague would also be discussing the possibility of restarting the Middle East peace process and supporting "Jordan's efforts to open up the political space".
Mr Hague is the highest-ranking Western politician to visit Tunisia since President Zine al-Abidine Ben Ali was toppled.
The end of Mr Ben Ali's 23-year rule last month sparked a wave of similar protests elsewhere in the Arab world, most prominently in Egypt, where 300 people are thought to have died in the unrest.
There have also been anti-government protests in Jordan, where King Abdullah dismissed his government, and in Yemen, where President Ali Abdullah Saleh announced he would not stand for re-election in 2013.
During his visit to Tunis on Tuesday, Mr Hague met leading figures from the interim government, including Prime Minister Mohammed Ghannouchi and International Co-operation Minister Mohamed Nouri Jouini.
At the talks, he pledged the UK's support for the democratic aspirations of the people of Tunisia and their desire for greater economic development and a more open political system.
The foreign secretary also announced the creation of the Arab Partnership Initiative, which will provide £5m of funding to support reform projects across the region in the areas of access to justice, freedom of expression, democratic institutions and civil society.
"It is a time of great opportunity for the Middle East. The courage, dignity and sacrifice of ordinary people in Tunisia in pursuit of universal freedoms that we take for granted has been inspiring," he said.
"Freedom of assembly, the rule of law, freedom of speech, and free and fair elections - these are inalienable rights that are the building blocks of free and open societies.
"The UK stands ready to support those across the region who aspire to greater economic development and more open political systems."
The BBC's James Robbins in Tunis says the UK shares Western nervousness that instability and chaos threaten the region unless there is genuine reform.
Mr Ghannouchi, a longtime ally of Mr Ben Ali who served as his prime minister for 11 years, has promised to hold elections within six months and said he will quit politics "in the shortest possible timeframe".
But pro-democracy protesters continue to demand that any members of the government who were part of the former regime step down now.
In a bid to counter the continuing unrest, on Sunday parliament banned the once all-powerful Democratic Constitutional Rally (RCD) party, but also granted decree powers to interim President Foued Mebazaa.
"Time is precious. Tunisia has real need of rule by decree to remove dangers," Mr Ghannouchi warned before the vote.
The Foreign Office said it was "the wrong time to go to Egypt given the talks between the government and opposition parties and the importance of not interfering".
Reporting on the foreign secretary's travel plans ahead of time is usually restricted for security reasons, but Mr Hague broke the embargo on his trip to Tunis by posting a message on his Twitter account on Monday.
"Heading to Tunisia to meet the new interim government & show UK support for the people of Tunisia & their democratic hopes," he wrote.