LONDON -- Prince William pledged to carry on the charitable legacy of his mother, the late Princess Diana, as he celebrated his 21st birthday on Saturday.

William's coming of age is accompanied by a host of new responsibilities for the second in line to the throne, but the prince is also planning on some fun, throwing an African-themed fancy dress party.

The royal birthday _ the only event on Britain's social calendar Saturday to rival the release of the latest Harry Potter book _ was also marked with the release of a rap-style poem by the country's poet laureate.

In an interview to mark the occasion, William said he was greatly influenced by visits to homeless hostels as a teenager with his mother, Princess Diana, who was killed in a Paris car crash in 1997.

"I learned a lot from it, more so now than I did at the time," William told Press Association, the British news agency. "It's made me aware, and I think homelessness is one of those topics that people kind of gloss over and don't really focus on. It is an important issue that needs to be understood and highlighted.

"My mother used her position very well to help other people, as does my father, and I hope to do the same," he said.

William defended 54-year-old father Prince Charles, who is often mocked by British tabloids.

"He does so many amazing things," William said. "I only wish people would see that more because he's had a very hard time and yet he's stuck it out and he's still very positive."

The prince said he would concentrate on completing the last two years of his four-year degree at St. Andrews University in Scotland before deciding how he would commit himself to charitable work.

The death of Diana heralded a new era of royal coverage. The press restrained its coverage of her young sons, and they have had much privacy during their educations.

But the royal family has responded to the intense interest around William's birthday by allowing a series of rare interviews and photo calls with the young prince.

Each new tidbit of information -- including the fact revealed in an interview last month that he prefers cider to beer -- has been seized on and analyzed as possible indicators of William's character.

On Thursday, "Wills" knocked back liqueurs and joked with photographers that they were trying to get him drunk as he toured Wales with his father.

On Friday, the nation saw the prince in TV footage joking with his father and being teased by his younger brother Prince Harry, 18, at a polo match.

More frivolity was expected later Saturday when William is guest of honor at an "Out of Africa" party for more than 300 family and friends at Windsor Castle.

William, who is teaching himself Swahili and has been on safari in Africa several times, said he wanted to avoid a "sterile" official function and have his family, including grandmother Queen Elizabeth II, let their hair down.

"I thought it would quite fun to see the family out of black tie and get everyone to dress up," said William, the picture of a modern royal himself in denim jeans and an open-necked shirt.

"My grandmother may be slightly apprehensive as to what she's going to wear, and what's going to happen, but she's very much looking forward to it. She's very positive towards the whole thing," he said.

On a more serious note, William's birthday brings the responsible position of Counselor of State. Along with five others, including his father, the prince is delegated powers to act on behalf of the queen, Britain's head of state.

Britain's poet laureate, Andrew Motion, gave the prince a gift of two poems _ one in rap form and one a sonnet.

Motion said the tribute, "On The Record," is a sort of literary double-sided LP to reflect William's role as a "new kind of royal figure," who also retains traditional responsibilities.

The rap version begins:

"Better stand back
Here's an age attack,
But the second in line
Is dealing with it fine.

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The sonnet ends:

That's what our `happy birthday' means today:
A wish that you'll be free to claim your life
While destiny connects with who you are -
A prince and yet familiar common clay;
Your father's heir but true to your own faith;
A mother's son and silvered by her star.